One of the more controversial of the effects claimed for marijuana is the production of
an "amotivational syndrome." This syndrome is not a medical diagnosis, but it
has been used to describe young people who drop out of social activities and show little
interest in school, work, or other goal-directed activity. When heavy marijuana use
accompanies these symptoms, the drug is often cited as the cause, but there are no
convincing data to demonstrate a causal relationship between marijuana smoking and these
behavioral characteristics.23 It is not enough to observe that a chronic
marijuana user lacks motivation. Instead, relevant personality traits and behavior of
subjects must be assessed before, as well as, after the subject becomes a heavy marijuana
user. Because such research can only be done on subjects who become heavy marijuana users
on their own, a large population study such as the Epidemiological Catchment Area study
described earlier in this chapter - would be needed to shed light on the relationship
between motivation and marijuana use. Even then, while a causal relationship between the
two could, in theory, be dismissed by an epidemiological study, causality not be proven.
- Measures of mood, cognition, and psychomotor performance should be incorporated into
clinical trials evaluating the efficacy of marijuana or cannabinoid drugs for a given
medical condition. Ideally, participants would complete mood assessment questionnaires at
various intervals throughout the day for a period prior to and at weekly intervals during
treatment, and, where appropriate, after the cessation of marijuana therapy. A full
psychological screening of research participants should be conducted to determine whether
there is an interaction between the mood altering effects of chronic marijuana use and the
psychological characteristics of the individual. Similarly, the cognitive and psychomotor
functioning of individuals should be assessed prior to and at regular intervals during the
course of a chronic regimen of marijuana or cannabinoid treatment to determine to what
extent tolerance develops to the impairing effects of marijuana and to monitor whether new
- When compared to changes produced by either placebo or an active control medication, the
magnitude of desirable therapeutic effects produced by marijuana could be determined, as
well as the frequency and magnitude of adverse psychological side effects. This would
allow a more thorough assessment of the risk:benefit ratio associated with the use of
marijuana for a given indication.
CONCLUSION: The psychological effects of cannabinoids, such as anxiety
reduction, sedation, and euphoria can influence their potential therapeutic value. Those
effects are potentially undesirable for certain patients and situations, and beneficial
for others. In addition, psychological effects can complicate the interpretation of other
aspects of the drug effect.
RECOMMENDATION: Psychological effects of cannabinoids such as anxiety reduction and
sedation, which can influence medical benefits, should be evaluated in clinical trials.
Physiological Harms: Tissue and Organ Damage
Many people who spoke to the IOM study team in favor of the medical use of marijuana
cited the absence of marijuana overdoses as evidence that it is safe. Indeed,
epidemiological data indicate that - for the general population - marijuana use is not
associated with increased mortality.(Sidney 1997a ) However, there are other serious
health outcomes to consider, and they are discussed below.
It is important to keep in mind that most of the studies that report physiological
harms resulting from marijuana use are based on the effects of marijuana smoking. Thus we
emphasize that the effects reported cannot be presumed to be caused by THC alone or even
in combination with other cannabinoids found in marijuana. It is likely that smoke is a
major cause of the reported effects. In most studies, the methods used make it impossible
to weigh the relative contributions of smoke versus cannabinoids.
- The relationship between marijuana and the immune system presents many facets, including
potential benefits and suspected harms. This section reviews the evidence on suspected
harms to the immune system caused by marijuana use.
- Despite the many claims that marijuana suppresses the human immune system, the health
impact of marijuana-induced immunomodulation is still unclear. Few studies have been done
with animals or humans to assess the effects of marijuana exposure on host resistance to
bacteria, viruses or tumors.
- Several approaches have been used to determine the effects of marijuana on the human
immune system, but each of these has serious limitations which will be discussed below.
assays from marijuana smokers
- One of the more common approaches has been to isolate peripheral blood leukocytes from
individuals who have smoked marijuana in order to evaluate the immune response of those
cells in vitro - most often by measuring mitogen-induced cell proliferation, a
normal immune response. Almost without exception, this approach has failed to demonstrate
any reduction in leukocyte function. The major problem with this approach is that after
drawing blood samples from the study subjects, the leukocytes need to be isolated from
whole blood before they are tested. This is done by high-speed centrifugation followed by
extensive washing of the cells, thus resulting in removal of the cannabinoid; perhaps for
that reason no adverse effects have been demonstrated in peripheral blood leukocytes from
marijuana Smokers. 74, 90, 122, 159
Leukocyte responses to THC
- Another approach is to isolate peripheral blood leukocytes from healthy control
individuals who do not smoke marijuana and then to measure the effect of THC on the
ability of these cells to proliferate in response to mitogenic stimulation in vitro.
However, it is notable that one significant difference between leukocytes isolated from a
marijuana smoker, as described above, and the in vitro studies in which THC has
been added directly to leukocyte cell cultures is in the cannabinoid composition.
Marijuana smoke contains many distinct cannabinoid compounds of which THC is just one.
Moreover, since the immunomodulatory activity of many of the other cannabinoid compounds
present in marijuana smoke has never been tested, and because it is now known that at
least one of those - cannabinol (CBN) - has greater activity on the immune system than the
central nervous system, 63
it is unclear whether the profile of activity
observed with THC accurately represents the effects of marijuana smoke on immune
competence. Likewise, it is unclear to what extent different cannabinoids in combination
exhibit additive, synergistic or antagonistic effects with respect to their
immunomodulatory activity. This issue is further complicated by the fact that leukocytes
express both types of cannabinoid receptors, CB1 and CB2
- One additional factor which may affect the immunomodulatory activity of cannabinoids in
leukocytes is metabolism. Since leukocytes have very low levels of
the cytochrome P-450 drug metabolizing enzymes 20, the metabolism of
cannabinoids is probably different between in vivo and in vitro exposure.
This last point is primarily pertinent to investigations of chronic and not acute
Human-derived cell lines
- A third approach for investigating the effects of cannabinoids on human leukocytes has
been to study human-derived cell lines. b As described above, the cell lines
are treated in vitro with cannabinoids to test their responses to different
stimuli. Although cell lines are a convenient source of human cells, the same problems
described above apply. Additionally, the cell lines might not be the same as the original
cells. For example, cell lines do not necessarily have the same number of cannabinoid
receptors as the original human cells.
- The most widely used approach is to evaluate the effects of cannabinoids in rodents,
using rodent-derived cells in vitro. The rationale for this approach is based on the fact
that the human and rodent immune systems are remarkably similar, and it is assumed that
the effects produced by cannabinoids on the rodent immune system will be similar to those
produced in humans. Although no significant species differences in immune system
sensitivity to cannabinoids have been reported, it is important to consider the
- The complete effect of marijuana smoking on immune function remains unknown. More
importantly, it is not known whether smoking leads to increased rates of infections,
tumors, allergies, or autoimmune responses. The problem is how to duplicate the
"normal" marijuana smoking pattern while at the same time removing other
potential immune modulating life-style factors such as alcohol and tobacco use.
Epidemiological studies are needed to determine whether marijuana users have a higher
incidence of diseases such as infections, tumors, allergies, and autoimmune disease.
Studies on resistance to bacterial and viral infection are clearly needed, and should
involve the collaboration of immunologists, infectious disease specialists, oncologists,
b Cell lines are created by removing cells from an organism and then
treating them so they are "immortalized,' meaning they will continue to divide
indefinitely in culture. Cellular processes can then be studied in isolation from their
- Tobacco is the predominant cause of lung diseases such as cancer and emphysema, and
marijuana smoke contains many of the same components as tobacco smoke. 7 Thus,
it is important to consider the relationship between habitual marijuana smoking and
certain lung diseases.
- Given a cigarette of comparable weight as much as four times the amount of tar can be
deposited in the lungs of marijuana smokers compared to tobacco smokers. 161
This is primarily due to the differences in filtration and smoking technique between
tobacco and marijuana smokers. Marijuana cigarettes usually do not have filters, and
marijuana smokers typically develop a larger puff volume, inhale more deeply, and hold
their breath several times longer than tobacco smokers. 118 However, a
marijuana cigarette smoked recreationally typically is not packed as tightly as a tobacco
cigarette, and thus the smokable substance is about half that in a tobacco cigarette. In
addition, tobacco smokers generally smoke considerably more cigarettes per day than do
Lymphocytes: T and B Cells
- Human studies of the effect of marijuana smoking on immune agents are not all consistent
with cannabinoid cell culture and animal studies. For example, antibody production was
decreased in a group of hospitalized patients who smoked marijuana for four days (twelve
cigarettes/day), but this decrease was seen in only one subtype of humoral antibody (IgG),
while two other subtypes (IgA and IgM) remained normal and one (IgE) was elevated. 107
Additionally, T cell proliferation was normal in the blood of a group of marijuana
smokers, although when evaluated more closely, there was an increase in one subset of T
cells 160 and a decrease in a different subset (CD8). 156 From these
studies it appears that marijuana use is associated with intermittent disturbances in T
and B cell function, but the magnitude is small and other measures are frequently normal. 86
- Alveolar macrophages are the principal immune-effector cells in the lung and are
primarily responsible for protecting the lung against infectious microorganisms, inhaled
foreign substances, and tumor cells. They are increased during tissue inflammation. In a
large sample of volunteers, habitual marijuana smokers had twice as many alveolar
macrophages as nonsmokers, and smokers of both marijuana and
tobacco had twice as many again. 11 Marijuana smoking also reduced the
ability of alveolar macrophages to kill fungus such as Candida aIbicans, c
pathogenic bacteria such as Staphylococcus aureus, and tumor target cells. This
reduction in ability to destroy fungal organisms was similar to that seen in tobacco
smokers. The inability to kill pathogenic bacteria was not seen in tobacco smokers. 10
Furthermore, marijuana smoking depressed pro-inflammatory cytokine production (such as
TNF-a and IL-6), but not Immunosuppressive cytokines. 10 Since cytokines are
important regulators of macrophage function, this marijuana-related decrease in
inflammatory cytokine production might be a mechanism whereby marijuana smokers are less
able to destroy fungal and bacterial organisms, as well as tumor cells.
- The inability of alveolar macrophages from habitual marijuana smokers without apparent
disease to destroy fungus, bacteria, and tumor cells, and to release pro-inflammatory
cytokines, suggests that marijuana might be an immunosuppressant with clinically
significant effects on host defense. Therefore, the risks of smoking marijuana should be
seriously weighed before recommending ifs use in any patient with pre-existing immune
deficits -- including AIDS patients, cancer patients, and those receiving
immunosuppressive therapies (for example, transplant or cancer patients).
- A number of animal studies have revealed certain respiratory tract changes and diseases
associated with marijuana smoking, while others have not. Extensive damage to the smaller
airways, which are the major site of chronic obstructive pulmonary disease (COPD), d
as well as acute and chronic pneumonia have been observed in various species exposed to
different doses of marijuana smoke. 41, 42, 127 In contrast, rats exposed to
increasing doses of marijuana smoke for one year did not show any signs of COPD, whereas
rats exposed to tobacco smoke did. 66
Chronic Bronchitis and Respiratory
- Human studies suggest that there is a greater chance of respiratory illness in
individuals who smoke marijuana. In a survey of outpatient medical visits in a large HMO,
marijuana users were more likely to seek help for respiratory illnesses than people who
smoked neither marijuana nor tobacco. 119 However, the incidence of
c A yeast infection that is particularly prevalent among people whose immune
systems are suppressed, such as in AIDS patients.
d A slow progressive obstruction of the airways, loss of their elasticity,
and loss of lung volume. Characterized by chronic shortness of breath, chronic bronchitis
and reduced oxygenation of blood.
seeking help for respiratory illnesses was not elevated for those who smoked marijuana
for 10 years or more when compared to those who smoked for less than 10 years. One
explanation for this is that people who experience respiratory symptoms are more likely to
quit smoking, and the people who continue to smoke represent a set of survivors who do not
develop or are indifferent to such symptoms. A limitation of this study is that no data
were available on the use of cocaine, which when used with marijuana could contribute to
the observed differences. Another limitation is that the survey relied on self-reporting,
and tobacco, alcohol and marijuana use might have been underreported (S. Sidney, IOM
- When marijuana smokers were compared to non-smokers and tobacco smokers in a group of
446 volunteers, 15-20 % reported symptoms of chronic bronchitis, including chronic cough
and phlegm production. 145 Twenty to 25 % of the tobacco smokers also reported
symptoms of chronic bronchitis. Despite a marked disparity in the amount of each substance
smoked per day (3-4 joints of marijuana versus more than 20 cigarettes of tobacco), the
difference in the percent of tobacco-smokers and marijuana-smokers experiencing symptoms
of chronic bronchitis was insignificant. 145 Similar findings were reported by
Bloom and coworkers, 15 and these investigators noted an additive effect of
smoking both marijuana and tobacco.
Bronchial Tissue Changes
- Habitual marijuana smoking is associated with changes in the lining of the human
respiratory tract. Many marijuana or tobacco smokers have increased redness (erythema) and
swelling (edema) of the airway tissues and increased mucus secretions. 43, 55
In marijuana smokers, the number and size of small blood vessels in the bronchial wall are
increased, tissue edema is present, and the normal ciliated cells e lining the
inner surface of the bronchial wall are largely replaced by mucus secreting goblet cells..
Moreover, the damage is even greater in people who smoke both marijuana and tobacco. 129
Overproduction of mucus by the increased numbers of mucus-secreting cells in the face of
diminished numbers of ciliated cells tends to leave cough as the only major mechanism to
remove mucus from the airways. This might explain the relatively high proportion of
marijuana smokers who complain of chronic cough and phlegm production. 147
- A 1998 study has shown that, compared to nonsmokers, both marijuana and tobacco smokers
have significantly more cellular and molecular abnormalities in bronchial epithelium
cells; those changes are associated with increased risk of cancer. 12 The
tobacco-only smokers in that study smoked an average of 25 cigarettes per day, whereas the
marijuana-only smokers smoked an average of 21 marijuana cigarettes per week. Despite the
fact that the marijuana smokers smoked many fewer
e Ciliated cells have hair-like projections that function to transport mucus
toward the mouth by rapid wave-like motion.
cigarettes, their cellular abnormalities were equivalent, or greater than, those seen
in tobacco smokers. This and earlier studies have shown than such abnormalities are
greatest in people who smoke both marijuana and tobacco, suggesting an additive effect of
marijuana and tobacco smoke on airway tissue. 12 43 55 Tenant and coworkers 149
found similar results in U.S. servicemen who suffered from respiratory symptoms and were
heavy hashish smokers. (Hashish is the resin from the marijuana plant).
Chronic Obstructive Pulmonary Disease
- In the absence of epidemiological data, indirect evidence, such as nonspecific airway
hyperresponsiveness and measures of lung function, offers an indicator of the
vulnerability of marijuana smokers to COPD. 153 For example, the methacholine
provocative challenge test, used to evaluate airway hyperresponsiveness, showed that
tobacco smokers develop more airway hyperresponsiveness. But no such correlation has been
shown between marijuana smoking and airway hyperresponsiveness.
- There are conflicting results as to whether regular marijuana use harms the small
airways of the lungs. Bloom found that an average of one joint smoked per day
significantly impaired the function of small airways. 15 On the other hand,
Tashkin and coworkers 145 did not observe such damage among heavier marijuana
users (3-4 joints per day for at least 10 years), but did note a narrowing of large,
central airways. Tashkin s long-term study, which adjusted for age-related decline in lung
function (associated with an increased risk for developing COPD), showed an accelerated
rate of decline in tobacco smokers, but not in marijuana smokers. 146 Thus the
question as to whether usual marijuana smoking habits are enough to cause COPD remains
- Chronic marijuana smoking might lead to acute and chronic bronchitis and extensive
microscopic abnormalities in the cells lining the bronchial passageways, some of which may
be pre-malignant. These respiratory symptoms are similar to that of tobacco smokers, and
the combination of marijuana and tobacco smoking augments these effects. At the time of
this writing, it has not been established whether chronic smoking marijuana causes COPD,
but there is probably an association.
- The relationship between marijuana smoking and the natural course of AIDS is of
particular concern because HIV patients are the largest group who report using
marijuana for medical purposes. Marijuana use has been linked to both increased risk of
progression to AIDS in HIV-seropositive patients, and to increased mortality in AIDS
- For reasons as yet unknown, marijuana use is associated with increased mortality among
men with AIDS, but not among the general population. 137 (The relative risk of
AIDS mortality for current marijuana users in this 12-year study was 1.90, indicating
that, compared to non-current marijuana users, almost twice as many marijuana users died
of AIDS.) Never married men used twice as much marijuana as married men and accounted for
83% of the AIDS deaths in the study. The authors of the study note that, while marital
status is insufficient to adjust for lifestyle factors -particularly, homosexual behavior
- a substantial proportion of the never married men with AIDS were likely homosexuals or
bisexuals. This raises the possibility that the association of marijuana use with AIDS
deaths might be related to indirect factors, such as use of other drugs or high risk
sexual behavior, both of which increase risks of infection to which AIDS patients are more
susceptible. The higher mortality of AIDS patients who were current marijuana users also
raises the question as to whether this was because patients increased their use marijuana
at the end-stages of the disease to treat their symptoms. However, the association between
marijuana use and AIDS deaths was similar even when the subjects who died earliest in the
first 5 years of this 12 year study, and who were presumably the most sick, were excluded
from the analysis. In sum, it is premature to conclude what the underlying causes of this
association might be.
- For the general population, the risk of mortality associated with marijuana use was
lower than that associated with cigarette smoking, and tobacco smoking was not an
independent risk factor in AIDS mortality in this study. Interestingly, the authors of
this study concluded that therapeutic use of marijuana did not contribute to the increased
mortality among men with AIDS.
- Marijuana use has been associated with a higher prevalence of HIV seropositivity in
cross-sectional studies,(Ostrow 1994) 83 but the relationship of marijuana to
the progression to AIDS in HIV-seropositive patients is a reasonable question. It remains
unclear whether marijuana smoking is an independent risk factor in the progression of AIDS
in HIV-seropositive men. Marijuana use did not increase the risk of AIDS in
HIV-seropositive men in the Multicenter AIDS Cohort Study, in which 1,795 HIV-seropositive
men were studied for 18 months, 83 or in the San Francisco Men's Health Study,
in which 451 HIV-seropositive men were studied for 6 years. 34 In contrast, the
Sydney AIDS Project in Australia (386 HIV seropositive men were studied for 12 months) 151
reported that marijuana use was associated with increased risk of progression to AIDS. But
the results of the 1988 Sydney study are less reliable than the other two studies
described. For one, this was the shortest of the studies; and, second, according to the
1993 definition of
AIDS, many of the subjects probably already had AIDS at the beginning of the study. f
- The most compelling concerns regarding marijuana smoking in HIV/AIDS patients are the
possible effects of marijuana on human immunity. 110 Reports of Opportunistic
fungal and bacterial pneumonia in patients with AIDS who used marijuana suggest that
marijuana smoking either suppresses the immune system, 33 or that it exposes
patients to an added burden of pathogens. 21 In sum, patients with preexisting
immune deficits due to AIDS should be expected to be vulnerable to serious harms caused by
smoking marijuana. The relative contribution of marijuana smoke versus THC or other
cannabinoids is not known.
- The gas and tar phases of marijuana and tobacco smoke contain many of the same
compounds. Furthermore, the tar phase of marijuana smoke contains higher concentrations of
polycyclic aromatic hydrocarbons (PAHs), such as the carcinogen benzopyrene. The higher
content of carcinogenic PAHs in marijuana tar and the greater deposition of this tar in
the lung might act in conjunction to amplify the exposure of the marijuana smoker to
carcinogens. For these reasons the carcinogenicity of marijuana smoke is an important
- Compared to studies on tobacco smoke, it is more difficult to collect the
epidemiological data necessary to establish or refute the link between marijuana smoke and
cancer. Far fewer people smoke only marijuana than who smoke only tobacco, and marijuana
smokers are more likely to underreport their smoking.
- Several case-series suggest that marijuana might play a role in the development of human
respiratory cancer. Those reports indicate an unexpectedly large proportion of marijuana
users among cases of lung cancer 140, 148 and cancers of the upper
aerodigestive tract - that is, the oral cavity, pharynx, larynx, and esophagus - that
occur before age 45 years. 36, 39, 148 Respiratory tract cancers associated
with heavy tobacco and alcohol consumption are not usually seen before the age of 60
years, 153 and the occurrence of such cancers in marijuana users younger than
60 years suggests that long-term marijuana smoking potentiates the effects of other risk
factors, such as tobacco smoking, and is a more potent risk factor than tobacco and
alcohol use in the early development of respiratory cancers. Unfortunately, most studies
lack the necessary comparison groups to calculate the isolated effect of marijuana use on
cancer risk. Many marijuana smokers also smoke tobacco, so when studies lack information
regarding cigarette smoking status, there is no way to separate the effects of marijuana
smoke from tobacco smoke.
f In 1993, the diagnosis of AIDS was expanded to include anyone with a CD4
count less than 200. Prior to 1993, this alone would have been insufficient for a
diagnosis of AIDS.
- As of this writing, Sidney and coworkers 138 have conducted the only
epidemiological study to evaluate the association between marijuana use and cancer. The
study included a cohort of approximately 65,000 men and women between the ages of 15 and
49 years. Marijuana users were defined as those who had used marijuana on six or more
occasions. Among the 1,421 cases of cancer in this cohort, marijuana use was associated
only with an increased risk of prostate cancer in men who did not smoke tobacco. In these
relatively young HMO clients, no association was found between marijuana use and other
cancers, including all tobacco-related cancers, colorectal or melanoma. The major
limitation associated with interpreting this study is that the development of lung cancer
requires a long exposure to smoking, and most marijuana users quit before this level of
exposure is achieved. Additionally, marijuana use has only been widespread in the U.S.
since the late 1960s; therefore, despite the large cohort size there might not have been a
sufficient number of heavy and/or long-term marijuana smokers to observe an effect.
and Molecular Studies
- In contrast to clinical studies, the evidence from cellular and molecular studies
provides strong evidence that marijuana smoke is carcinogenic. Cell culture studies
implicate marijuana smoke in the development of cancer. Prolonged exposure of hamster lung
cell cultures to marijuana smoke led to malignant transformations, 93 and human
lung explants exposed to marijuana smoke resulted in chromosomal and DNA alterations. 153
Additionally, the tar from marijuana smoke induced mutations similar to those produced by
tar from the same quantity of tobacco in a common bacterial assay for mutagenicity. 157
- Molecular studies also implicate marijuana smoke as a carcinogen. Protooncogenes and
tumor suppressor genes are a group of genes that affect cell growth and differentiation.
Normally, those genes code for proteins that control cellular proliferation. Once mutated
or activated, these genes produce proteins which cause cells to multiply rapidly and out
of control, resulting in tumors or cancer. g When the production of these
proteins was evaluated in tissue biopsies taken from marijuana, tobacco, marijuana plus
tobacco smokers, and non-smokers, respectively, two of them (EGFR and Ki-67) were markedly
increased in the marijuana smokers compared to non-smokers and tobacco smokers. Moreover,
the effects of marijuana and tobacco were additive. 130 Thus, in relatively
young smokers of marijuana, and
g Some of the genes involved in the development of lung cancer include those
that encode for Ki-67 (a nuclear proliferation protein responsible for cell division), the
p53 tumor suppressor (a protein that normally suppresses cell growth), and epidermal
growth factor receptor (EGFR) (a receptor found on a variety of cell types, especially
epithelia, and which promotes cellular growth and proliferation when bound to epidermal
particularly in those who smoke both marijuana and tobacco, marijuana is implicated as
a risk factor for lung cancer.
- DNA alterations are known to be early events in the development of cancer, and have been
observed in the lymphocytes of pregnant marijuana smokers, as well as in those of their
newborns. 4 This is an important study because the investigators were careful
to exclude tobacco smokers - a problem in previous studies that cited mutagenic effects of
marijuana smoke. 26, 53, 62, 141 These same investigators found similar effects
in previous studies among tobacco smokers, 5, 6 so the effects cannot be
attributed solely to THC or other cannabinoids. Although it can only be determined by
experiment, it is likely that the smoke contents - other than cannabinoids - are
responsible for a large part of the mutagenic effect.
- Preliminary findings suggest that marijuana smoke activates cytochrome P4501A1 (CYPlA1),
the enzyme that converts PAHs, such as benz[a]pyrene, into active carcinogens 98
Bronchial epithelial cells in tissue biopsies taken from marijuana smokers show more
binding to CYP1A1 antibodies than do comparable cells in biopsies from nonsmokers
(D.Tashkin, IOM workshop). This suggests that there is more of the carcinogen-producing
enzyme, CYP1A1, itself in bronchial cells of marijuana smokers, but different experimental
methods will be needed to definitely establish that possibility.
- At this time, there is no conclusive evidence that marijuana causes cancer in humans,
including cancers usually related to tobacco use. However, cellular, genetic and human
studies all suggest that marijuana smoke maybe an important risk factor for the
development of respiratory cancer. More definitive evidence that habitual marijuana
smoking does or does not lead to respiratory cancer awaits the results of well-designed
case-controlled epidemiological studies. It has been 30 years since the initiation of
widespread marijuana use among young individuals in our society, and such studies should
now be feasible.
- The following studies or activities would be useful in providing data that could more
precisely define the health risks of smoking marijuana.
- 1. Case-control studies to determine whether marijuana use is associated with an
increased risk of respiratory cancer: Despite the lack of compelling epidemiological
evidence to date, findings from the biochemical, cellular, immunologic, genetic, tissue
and animal studies cited above strongly suggest that marijuana is a risk factor for human
cancer. What is required to address this hypothesis more convincingly is a
population-based case-controlled study of sufficiently large numbers of lung cancer cases
and cases of upper aerodigestive tumors (cancers of the oral cavity and pharynx, larynx
and esophagus), as well as non-cancer controls, to demonstrate a statistically significant
association, if one exists. Because of the long time period required for induction of
human carcinomas and the infrequent use of marijuana in the general U.S. population prior
to 1966, no
epidemiological studies so far have been extensive enough to adequately measure the
association between marijuana and cancer. However, epidemiological investigation of this
association is probably possible now, since approximately 30 years have elapsed since the
start of widespread marijuana use in the U.S. among teenagers and young adults.
- 2. Molecular markers of respiratory cancer progression in marijuana smokers. If
an epidemiological association between marijuana use and risk of respiratory cancer is
demonstrated, then studies are warranted to explore the presence of molecular markers that
may be predictive of genetically increased risk of carcinogenesis in marijuana users, such
as TP53, p16, NATZ and GSTML.
- 3. Prospective epidemiological studies of populations with HIV-seropositivity or at
high risk for HIV infection. Because HIV/AIDS patients are largest group that reports
smoking marijuana for medical purposes and they are particularly vulnerable to
immunosuppressive effects, there is a pressing need for a better understanding of the
relative risk and rewards of smoking marijuana. Such studies should include history of
marijuana use in the analysis of potential risk factors for seroconversion and acquisition
of opportunistic infections/progression to AIDS. h These studies could be
carried out in the context of any federally approved clinical trials of medicinal
marijuana in immunocompromised patients and should provide a sufficiently long period of
follow-up to capture any potential adverse events.
- 4. Regularized recording of marijuana use by patients. Although marijuana is the
most commonly used illicit drug, medical providers often do not question patients about
marijuana use and rarely document its use. 101 Among 452 Kaiser Permanente
patients who reported daily or almost daily marijuana use, physicians recorded marijuana
use in only 3 % of their medical records (S. Sidney, IOM workshop).
- 5. Additional cellular, animal and human studies to investigate further the impact of
THC and marijuana on immune function, including their effects on proinflammatory
versus immunosuppressive cytokines, and on the function of leukocytes that present antigen
to T cells.
- The question that needs to be addressed is whether THC or marijuana is a risk factor for
HIV infection, for progression to more severe stages of AIDS, or for opportunistic
infection among HIV-positive patients. Studies are needed to determine the effects of
marijuana use on the function of alveolar macrophages. It would be important to compare
the HIV infectivity and replication of alveolar macrophages harvested from habitual
marijuana users with those harvested from non- or infrequent marijuana users. Cell culture
studies could be used to compare the
h A prospective study is one in which a group of subjects is
identified and then studied over the course of time. Such a study allows an experimenter
to balance different factors that may contribute to the study outcome. For example, age,
family history, and smoking are risk factors for lung cancer. In a prospective study these
factors can be balanced to measure how much smoking increases the risk of lung cancer. A retrospective
study is one in which people with a particular disease are identified and their
histories are studied. Such studies are easier and less expensive to conduct, but they
generally lack the explanatory power of prospective studies.
susceptibility of HIV-infected alveolar macrophages to additional infection with
Opportunistic pathogens. Similarly, further studies on cell cultures of peripheral blood
mononuclear cells could be used to assess the impact of exposure to THC on HIV infectivity
- Marijuana smoke and oral THC can cause tachycardia (elevated heart rate) in humans,
usually 20 - 100 % above baseline. 56, 84 The increase in heart rate is
greatest in the first 10-20 minutes after smoking, and decreases sharply and steadily;
depending on whether smoked marijuana or oral THC are used, these side effects can last 3
or 5 hours, respectively. 67, 94 In some cases, blood pressure can be increased
while a person is in a reclining position, but can decrease inordinately on standing,
resulting in postural hypotension. i In contrast to acute administration of
THC, chronic oral ingestion of THC reduces heart rate in humans. 13
- In animals, THC decreases heart rate and blood pressure. 56, 155 However,
most such animals studies are conducted in anaesthetized animals, and anesthesia causes
hypertension. Thus, those studies should be interpreted as reports on the effects of
cannabinoids in hypertensive subjects. The results of the animal and human studies are
consistent with the conclusion that cannabinoids are hypotensive at high doses in animals,
as well as humans (see 1998 review by Wagner and coworkers). 155
- Tolerance can appear after a few days of frequent daily dosing (2 - 3 times per day) of
oral THC or marijuana extract with heart rate slowing, reclining blood pressure falling,
and postural hypotension disappearing. 72 Thus, the intensity of these effects
depends on the frequency of use, dose, and even body position.
- The cardiovascular changes have not been a health problem for healthy, young users of
marijuana or THC. However, such changes in heart rate and blood pressure could present a
serious problem for older patients, especially those with coronary artery or
cerebrovascular disease. Since cardiovascular diseases are the leading causes of death in
the United States (coronary heart disease is first, stroke is third), any impact of
marijuana use on cardiovascular disease could have a substantial impact on public health
(S. Sidney, IOM workshop). The magnitude of this impact remains to be determined as
chronic marijuana users from the late 1960s enter the age where coronary artery and
cerebrovascular diseases become common. Additionally, smoking marijuana is known to
decrease maximal exercise performance. This, along with the increased heart rate, could
theoretically induce angina (S. Sidney, IOM workshop). Therefore, this raises the
possibility that patients with symptomatic coronary artery disease should be advised not
to smoke marijuana, and THC might be contraindicated in patients with restricted
i Decreased blood pressure due to changing posture from a lying or sitting
position to a standing position, which can cause dizziness and faintness.
Marijuana and THC can inhibit many reproductive functions on a short-term basis. In
both male and female animals, THC injections suppress reproductive hormones and behavior.106,
158 Studies have consistently shown that injections of THC result in rapid,
dose-dependent suppression of serum luteinizing hormone (LH) levels. 69 (LH is
the pituitary hormone that stimulates release of the gonadal hormones, testosterone and
estrogen.) Embryo implantation also appears to be inhibited by THC. But it does not
necessarily follow from this that marijuana use will interfere with human reproduction.
With few exceptions, the animal studies are based on acute (i.e., single injections) or
short-term treatments (i.e., THC injections given over a series of days). The results are
generally observed for only several hours, or sometimes in females for only one ovulatory
Acute treatments of cannabinoids, including THC, CBD, and cannabinol, and anandamide
can decrease the fertilizing capacity of sea urchin sperm. 134, 135, 136 While
the sea urchin is only a distant relative of humans, the cellular processes that regulate
fertilization are similar enough that one can expect a similar effect in humans. However,
the effect of cannabinoids on the capacity of sperm to fertilize eggs is reversible, and
is observed at 6-100 ÁM concentrations, 135, 136 which are higher than those
likely to be experienced by marijuana smokers. At the same time, the presence of
cannabinoid receptors in sperm suggests the possibility of a natural role for anandamide
in modulating sperm function during fertilization. However, it remains to be determined
whether smoked marijuana or oral THC taken in prescribed doses has a clinically
significant effect on the fertilizing capacity of human sperm.
- Exposure to THC in utero can result in long-term changes. Many in utero
effects interfere with embryo implantation (see review by Wenger and coworkers 158).
Exposure to THC: close to the time of, or shortly after, their birth can also result in
impaired reproductive behavior in adult mice: females are slower to show sexual
receptivity and males are slower to mount. 106
- Although THC can act directly on endocrine tissues, such as the testes or ovaries, it
appears to affect reproductive physiology through its actions on the brain, but somewhere
other than the pituitary. Some, but not all, of the effects of THC are through its effect
on stress hormones such as cortisol. 69
- The few human studies are consistent with the acute animal studies: THC inhibits
reproductive functions. However, studies of men and women who use marijuana regularly have
yielded conflicting results, and show either depression of
reproductive hormones, effect, or only a short-term effect. Overall, the results from
human studies are consistent with the hypothesis that THC inhibits LH on a short-term
basis, but not in long-term marijuana users. In other words, long-term users develop
tolerance to the inhibitory effect of THC on LH. The results in women are similar, with
the added consideration of the menstrual cycle; the acute effects of THC appear to vary
with cycle stage. THC appears to have little effect during the follicular phase (the phase
after menses and before ovulation), and to inhibit the LH pulse during the luteal phase
(the phase after ovulation and before menses).102 In brief, although there are
no data on fertility per se, marijuana or THC would likely decrease human fertility - at
least in the short-term-for both men and women. And it is reasonable to predict that THC
can interfere with early pregnancy, particularly implantation of the embryo. Like tobacco,
marijuana smoke is highly likely to be harmful to fetal development and should be avoided
by pregnant women as well as those who might become pregnant in the near future.
Nevertheless, although fertility and fetal development are important concerns for many,
they are unlikely to be of much concern to people with seriously debilitating or
life-threatening diseases. The well-documented inhibition of reproductive functions by THC
is thus not a serious concern for evaluating the short-term medical use of marijuana or
Developmental impact of use during pregnancy
- Among the studies that have investigated the relationship between prenatal marijuana
exposure and birth outcome, the results have been inconsistent (reviewed by Cornelius and
coworkers 30 in 1995). Except for adolescent mothers, there is little evidence
that gestation is shorter in mothers who smoke marijuana. 30 Several studies of
women who smoked marijuana regularly during pregnancy show that they tend to give birth to
lower weight babies.46, 64 Mothers who smoke tobacco also give birth to lower
weight babies, and the relative contributions of smoking versus THC are not known from
- Babies born to mothers who smoked marijuana during pregnancy weighed, on average, 3.4
ounces less than babies born to the study's control group of mothers who did not smoke
marijuana; there was no significant difference in either gestational age or frequency of
congenital abnormalities.163 These results were based on women whose urine
tests indicated recent marijuana. However, when the analysis was based only on
self-reports of marijuana use (without verification by urine tests), there was no
difference between the weight of babies born to women who reported themselves as marijuana
smokers and those born to women who reported they did not smoke marijuana. This raises an
important concern about the methods used to measure the effects of marijuana smoking in
any study, and perhaps even more so in studies on the effects of marijuana during
pregnancy when subjects might be even less likely to admit to smoking marijuana. (The
study was conducted in the last trimester of pregnancy, and there was no information about
the extent of marijuana use earlier in the pregnancy).
- For most of these studies, much of the harms associated with marijuana use are
consistent with those associated with tobacco use, and smoking is a significant
factor so the contribution of cannabinoids cannot be confirmed. However, Jamaican women
who use marijuana rarely smoke it, but instead prepare it as tea. 37 In a study
of neonates born to Jamaican women who either did or did not ingest marijuana during
pregnancy, there was no difference in neurobehavioral assessments made at 3 days after
birth and at one month. 38 A limitation of this study is that there was no
direct measure of marijuana use. Estimates of marijuana use were based on self reports,
which might be more accurate in Jamaica than in the U.S. since there is less social stigma
associated with marijuana use in Jamaica, but are nonetheless less reliable than direct
- Newborns of mothers who smoke either marijuana or tobacco have significantly higher
mutation rates than those of non-smokers. 4, 5 Since 1978, the Ottawa Prenatal
Prospective Study has been measuring the cognitive functions of children born to mothers
who smoked marijuana during pregnancy. 47 Children of mothers who smoked either
moderately (1-6 marijuana cigarettes per week) or heavily (more than 6 marijuana
cigarettes per week) have been studied from age four days to 9-12 years. It is important
to keep in mind that studies like this provide important data about the risks associated
with marijuana use during pregnancy, but they do not establish the causes of any such
- The children in the different marijuana exposure groups showed no lasting differences in
global measures of intelligence such as language development, reading scores, and visual
or perceptual tests. Moderate cognitive deficits were detectable among these children when
they were four days old and again at four years, but these deficits were no longer
apparent at five years.
- Prenatal marijuana exposure was not, however, without lasting impact. By comparison, at
both ages 5-6 and 9-12, children in the same study who were prenatally exposed to tobacco
smoke scored significantly lower on tests of language skills and cognitive functioning.48
In another study, 49, 50 nine-to-twelve years olds who were exposed to
marijuana prenatally scored lower than control subjects on tasks associated with
"executive function," a term used by psychologists to describe an individual's
ability to plan ahead, anticipate, and suppress behaviors that are incompatible with a
current goal. 50 This was reflected in how the mothers described their
children. The mothers of the marijuana-exposed children were more likely to describe their
offspring as hyperactive or impulsive than did mothers of control children. This
alteration in executive function was not seen in children born to tobacco smokers. The
underlying causes might be the marijuana exposure, or might be more closely related to the
reasons underlying their mothers' use of marijuana during pregnancy.
- Mice born to dams injected with the endogenous cannabinoid, anandamide, during the last
trimester of pregnancy also showed delayed effects. No effect of anandamide treatment
during pregnancy was detected until the mice were adults (40 days of age), at which time
they showed behavioral changes that are common to the effects of other psychotropic drugs
or prenatal stress. 45 As with the children born to mothers who smoked
marijuana, it is not known what aspect of the treatment caused the effect. The dams might
have found the dose (20 mg/kg of body weight) of
anandamide aversive, in which case, the effect could have resulted from generalized
stress, as opposed to a cannabinoid-specific effect. Either case is possible.
- Despite the uncertainty as to the underlying causes of impact of prenatal exposure to
cannabinoid drugs, it is, nonetheless, prudent to advise against smoking marijuana during
Summary and Conclusions
- This chapter summarizes the harmful effects of marijuana to the individual user and, to
a lesser extent, to society. The harmful effects to individuals were considered from the
perspective of possible medical use of marijuana, and can be divided into acute and
chronic effects. The vast majority of data on harmful effects of marijuana is based on
smoked marijuana, and except for the psychoactive effects that can be reasonably
attributable to THC, it is not possible to distinguish the drug effects from the effects
of inhaling smoke of burning plant material.
- For most people, the primary adverse effect of acute marijuana use is diminished
psychomotor performance, it is inadvisable to operate any equipment that might put the
user or others in danger (such as driving and operating or monitoring complex equipment
under the influence of marijuana). While most people can be expected to show impaired
performance of complex tasks, a minority experience dysphoria. Individuals with or at risk
of psychiatric disorders (including substance dependence) are particularly vulnerable to
developing marijuana dependence and marijuana use would be generally contraindicated in
those individuals. The short term immunosuppressive effects are not well-established and,
if they exist at all, are not likely great enough to preclude a legitimate medical use.
The acute side effects of marijuana use are within the risks tolerated for many
- The chronic effects of marijuana are of greater concern for medical use and fall
into two categories: the effects of chronic smoking, and the effects of THC. Marijuana
smoke is like tobacco smoke in that it is associated with increased risk of cancer, lung
damage, and poor pregnancy outcomes. Smoked marijuana is unlikely to be a safe medication
for any chronic medical condition. The second category is that associated with dependence
on the psychoactive effects of THC. Despite past skepticism, it has been established that,
although it is not common, a vulnerable subpopulation of marijuana users can develop
dependence. Adolescents, particularly those with conduct disorders, individuals with
psychiatric disorders, or problems with substance abuse appear to be at greater risk for
marijuana dependence than the general population.
- As a cannabinoid drug delivery system, marijuana cigarettes are not ideal since they
deliver a variable mixture of cannabinoids, as well as a variety of other
biologically-active substances, not all of which are desirable or even known. Unknown
substances include possible contaminants such as fungus or bacteria
- Finally, there is the broad social concern that sanctioning the medical use of marijuana
might lead to an increase in its use among the general population. At this point there are
no convincing data to support this concern. The existing data are
consistent with the idea that this would not be a problem if the medical use of
marijuana were as closely regulated as other medications with abuse potential, but we
acknowledge that there are no data that directly address this question. Even if there were
evidence that the medical use of marijuana would decrease the perception that it can be a
harmful substance, this is beyond the scope of laws regulating the approval of therapeutic
drugs. Those laws concern scientific data concerning safety and efficacy drugs for
individual use, and do not address perceptions or beliefs of the general population.
- Marijuana is not a completely benign substance. It is a powerful drug with a variety of
effects. However, except for the harms associated with smoking, the adverse effects of
marijuana use are within the range tolerated for other medications. Thus the safety issues
associated with marijuana do not preclude certain medical uses. But the question remains:
is it effective? This topic is covered here in two chapters: chapter 2 summarizes what has
been learned about the biological activity of cannabinoids in the past fifteen years from
research in the basic sciences, chapter 4 reviews the clinical data on the effectiveness
of marijuana and cannabinoids for the treatment of a variety of medical conditions.
- Three factors influence the safely of marijuana or cannabinoid drugs for medical
uses: the delivery system, the use of plant material, and the side effects of cannabinoid
drugs. (l) Smoking marijuana is clearly harmful, and especially for chronic
conditions, and is not an ideal drug delivery system. (2) Plants are of uncertain
composition which render their effects equally uncertain, hence an undesirable medication.
(3) The side effects of cannabinoid drugs fall within the acceptable risks for approved
medications. Indeed, some of the 'side effects', such as anxiety reduction and sedation,
might be desirable for certain patients. As with many medications, there are people for
whom they would likely be contraindicated.
CONCLUSION: Present data on drug use progression neither support nor refute the
suggestion that medical availability would increase drug abuse. However, this question is
beyond the issues normally considered for medical uses of drugs, and should not be a
factor in evaluating the therapeutic potential of marijuana or cannabinoids.
CONCLUSION: A distinctive marijuana withdrawal syndrome has been identified, but
it is mild and short-lived. The syndrome includes restlessness, irritability, mild
agitation, insomnia, sleep EEG disturbance, nausea, and cramping.
CONCLUSION: Numerous studies suggest that marijuana smoke is an important risk
factor in the development of respiratory disease.
RECOMMENDATION: Studies to define the individual health risks of smoking marijuana
should be conducted, particularly among populations in which marijuana use is prevalent.
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