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Major Studies of Drugs and Drug Policy
Canadian Senate Special Committee on Illegal Drugs
Volume I - General Orientation

Chapter 7 - Cannabis: Effects and Consequences

Effects and consequences of cannabis: what we were told


During the hearings, many witnesses told us what they knew about the effects of cannabis. Some of this knowledge came from their own research work. Other knowledge came from their professional experiences. And lastly, other knowledge was either their interpretation of scientific literature or anecdotes. In this section, we will not make distinctions between the testimony and we will not evaluate its validity. We only want to highlight the richness, as well as the complexity, of what we were told.


Message number one is that drugs, including cannabis, are harmful. (…) There is considerable misinformation about the physiological consequences of cannabis use. There is no doubt that heavy use has negative health consequences. The most important are in the following areas: respiratory damage, physical coordination, pregnancy and postnatal development, memory and cognition, and psychiatric effects. (...) [1][7]


Generally, marijuana (cannabis) and its derivative products are described in this context to distance the drug from the recognized harm associated with other illegal drugs. This has been a successful yet dangerous approach and contributes to the misinformation, misunderstanding and increasing tolerance associated with marijuana use. Marijuana is a powerful drug with a variety of effects. Marijuana users are subject to a variety of adverse health consequences that include respiratory damage, impaired physical coordination, problem pregnancy and postnatal deficits, impaired memory and cognition, and psychiatric effects. Marijuana use is associated with poor work and school performance and learning problems for younger users. Marijuana is internationally recognized as a gateway drug for other drug use. Risk factors for marijuana dependence are similar to those of other forms of drug abuse. (…) It was the consensus of the international community to put marijuana and other substances under international control. That decision was based on evidence of its harmfulness to human health and its dependence potential. [2][8]


I wish to briefly review two of what I believe are fairly well-established, harmful effects of marijuana, and a number of other areas where there is considerable contention. (…) By far the most consistent and clear-cut effect of marijuana is disruption of short-term memory. Short-term memory is usually described as "working" memory. It refers to the system in the brain that is responsible for short-term maintenance of information needed for the performance of complex tasks that demand planning, comprehension and reasoning. The relatively severe impairment of working memory may help to explain why, during the marijuana high, subjects have difficulty maintaining a coherent train of thought or conversation. (…) Obviously this is relevant if you are going to school stoned. (…) It is becoming increasingly clear that cannabis is a drug on which regular users become dependent, and that this adversely affects large numbers of people. [3][9]


Marijuana has been shown to be associated with reckless drivers and motor vehicle accidents. Evidence suggests that marijuana may contribute to an appreciable number of traffic deaths and injuries in Canada. It has been shown to negatively affect the academic and social development of some adolescents. Marijuana can cause emotional and medical problems. Chronic use may be associated with lung diseases such as bronchitis, emphysema and cancer. A psychosis may develop in some individuals while other psychiatric symptoms such as anxiety, low mood, depression and panic do occur. Marijuana is known to be addictive. Although the rate of addiction varies, it is between 5 per cent and 10 per cent. I should like to stress that addiction is a disease and marijuana has the potential to be addictive to a genetically predisposed group of individuals. [4][10]


The evidence was that 95 per cent of the marijuana users in Canada are low, occasional, moderate users. Their consumption of marijuana does not impact on their health as long as they are healthy adults. The other 5 per cent are chronic users, people who smoke one or more marijuana cigarettes per day. If they continue to do that, they will ultimately get chronic bronchitis from the smoking process. The same would be true if they were to roll up the grass off their lawns and smoke that. They would inhale heated material over their large airways and cause damage to them. There were three primary vulnerable groups: pregnant women, which we submit is something between the woman and her doctor; the mentally ill, particularly paranoid schizophrenics (…) then, most importantly, immature youth. Young people who become involved with marijuana - particularly on a regular basis - seem to suffer from a disruption of their studies and the maturation process. As is the case with most intoxicants, it is recommended that they not become involved with them until they have matured. [5][11]


I have one resource from the Center for Substance Abuse Prevention in the U.S., where recent marijuana research and a number of studies indicate some of the risks. We already know and accept that cannabis has negative effects on many systems - respiratory, motor skills, memory and immune - and that it creates drug dependency and tension. In addition, we now know from numerous research studies that there is a definite and acute withdrawal syndrome associated with chronic cannabis use. (…) There is research that suggests there are effects on the developing fetus. (…) I will speak to visual scanning, specifically, attention dysfunction in the form of impaired visual scanning and related functioning. Visual scanning develops particularly in early adolescence, so earlier onset is associated with some concerns there. [6][12]


There are a number of negative health effects that have been created in the lab or have been observed with long-term users (…). There are, of course, health risks and negative health consequences with using the substance, but the majority of those risks only occur under specific circumstances. The majority of the risks are associated with long-term persistent and frequent use, and therefore must be understood as such. There is at this point agreement that the so-called dependence or withdrawal symptom may arise with heavy chronic users, but it is very much limited to that small population. (…) a seminal report by Hall and colleagues from Australia (…) concluded that the major risks of cannabis use can be significantly reduced by avoiding driving under the influence, by avoiding chronic and daily use, and by avoiding deep inhalation. These were the key factors that allowed us to avoid many of the major harms and risks associated with it. [7][13]


In any event, we are talking about plant derivatives that contain a number of psychoactive alkaloids. The psychoactive effects are predominantly of mild euphoria and time distortion, though disorientation and panic attacks may occur. The appreciation of music, art and food are said to be enhanced, as is appetite, and this later function seems important for one of the claimed medical benefits in offsetting the effects of the chronic wasting syndrome in AIDS and the prolonged nausea that accompanies chemotherapy. (…) Because the drug is usually smoked, it has acute and chronic effects that are shared with tobacco. These include airway irritation, cough, and probably with chronic use, bronchitis, chronic obstructive pulmonary disease, and lung and pharyngeal cancers. Its impact on the immune system is generally to impair the function of the immune system, but the impact on human health of this impairment is probably minor. (…) The effects of cannabis consumption on reproductive health are negative in animal studies. (…) This obviously has some relevance to human health. However, human studies have yet to show any measurable adverse impact beyond some evidence of adverse behavioural and developmental impacts on the children of mothers who smoked cannabis heavily during pregnancy. (…) The impact of cannabis on cognition is well documented. Short-term memory is adversely affected and chronic use may lead to chronic measurable defects in cognitive functioning. However, this may be more the result of persistent chronic intoxication than impairment in the substance and the working of the brain. Psychomotor skills are adversely affected by cannabis use. Driving or operating heavy machinery when intoxicated is contraindicated. Again, in contradistinction to alcohol, cannabis intoxication tends to slow drivers down rather than increase their speeds. Similarly, cannabis smokers tend not to be involved in acts of physical violence and aggression, and violence and aggression when intoxicated is reportedly very rare. Cannabis use may provoke schizophrenic symptoms in those with active schizophrenia or schizophrenic tendencies. Panic attacks and dysphoria are also mentioned in the literature. There is an amotivational syndrome described in the literature and cannabis is said to induce it, but most researchers have discredited that over the last decade. (…) Concerns have legitimately been raised about the effects of cannabis consumption on adolescent development. As use tends to peak in late adolescence, this is an important consideration. The adverse effects that have been noted include an association with risk of discontinuation of high school, job instability and progression to the use of harder drugs. The degree to which these associations are causal is very controversial. Alternative hypotheses are that cannabis use, like adolescent alcohol use, early onset of sexual activity, and tobacco smoking, are in fact markers for other risks of adverse social conditions (…) All researchers agree, however, that intoxication interferes with academic prowess. Recent studies seem to demonstrate measurable though reversible drops in IQ associated with heavy, persistent cannabis use and that engagement in illicit activities carries substantial risks, especially perhaps for youth whose connections to the school community are tenuous at best. [8][14]


I would like to first focus on the acute effects and then on the chronic effects. "Acute effects" are those effects that you experience during the course of action of a single dose. In the nervous system that includes a period of several hours in which (…) you become "chemically stupid." Side effects include decreased arousal and drowsiness, which acts together with the drowsiness produced by alcohol and other central nervous system depressants. Other side effects are impaired short-term memory, slowed reactions, less accuracy in test performance and less selectivity of attention. (…) Low doses generally produce the effects that cause people to like smoking pot. They include mild euphoria, relaxation, increased sociability and a non-specific decrease in anxiety. However, high doses produce a bad mood, anxiety and depression. There can be increased anxiety to the point of panic or even an acute toxic psychosis which, fortunately, is of very short duration and goes away when the drug effect wears off. High doses cause impaired motor coordination, unsteadiness of control and decreased muscle tone, which is therapeutically useful. (…) With low doses, perception is enhanced. That is part of the pleasure. In high doses, the same action produces sensory distortion, hallucinations and the acute toxic psychosis to which I have already referred. (…) It does not seriously affect the cardiovascular system. (…) As to chronic effects, in the central nervous system there is impaired memory, vagueness of thought, decreased verbal fluency, and learning deficits in chronic, heavy users. I emphasize "heavy" because the social user does not, by and large, show any significant health effects. Neither does the social user of alcohol. (…) These effects on cognitive functions fortunately tend to go away if the heavy user stops, for whatever reason. As long as use continues, there is a chronic intoxication, apathy, confusion, muddled thinking, depression, and sometimes paranoia. (…) Cannabis dependence, as defined in the conventional diagnostic criteria for dependence as set out in the latest edition of the American Psychiatric Association, or the equivalent publication of the World Health Organization, has been well documented in regular, heavy users. Numerous studies now show that a significant percentage of regular users are dependent. In some studies in Australia of long-term heavy users, mainly daily users for periods of 15, 17, 20 years, 60 per cent or more of them met the diagnostic criteria (…). Tolerance has been shown. By and large, it is not a terribly serious effect, and the physical withdrawal syndrome is not severe. Nevertheless, it is there, which indicates that physical dependence, in addition to psychological dependence, occurs as well. [9][15]


The long-term chronic effects of cannabis essentially cause the following symptoms: memory loss, faulty attention and concentration, a slow-motivation syndrome of passivity and low initiative, increased risk of respiratory disease, more specifically asthma, bronchitis and emphysema and a higher risk of cancer. (…) There may be hormone problems causing low fertility in men and women. In men, this can cause the development of breasts which is very unesthetic (…). Finally, in the long-term, it can also cause lower resistance to infectious disease. [10][16]

 As we can see, opinions sometimes agree and often differ. They agree at least on the nature of the consequences that may be of concern. One by one, we have seen effects that were physiological (risks of cancer, effects on reproduction and the immune system, deterioration of brain cells), effects that were psychological (amotivational syndrome, risks of psychosis, impaired cognitive function and memory in particular), and effects that were social (affecting the family and work, as well as the ability to drive vehicles and operate machinery). Opinions differ primarily on the scope of the conclusions that can be drawn from this knowledge. To what extent, in fact, can we generalize about the effects we observe in often small and rarely random samples of subjects? Also, to what extent can we generalize about the data on chronic users who represent – as we saw in the previous chapter – only a small percentage of cannabis users? And especially, to what extent does this data allow us to establish causal relationships?

The Committee also finds that most witnesses stressed the negative aspects and rarely the positive. However, if people use drugs in general, and cannabis in particular, surely it isn’t just to destroy themselves or because these drugs have only negative effects. Given the limitations of making any comparison between substances, we can still draw a parallel with alcohol: most of us know the pleasure of sharing a glass of wine with friends over a good meal, just as we also know the dangers of alcohol abuse and alcoholism. The Committee also notes that it is difficult, even for the most experienced researchers, to sift through the knowledge without assigning it a valence relative to the direction public policy should take. The same knowledge may be interpreted negatively here and more moderately there, based on the interpreter’s preconceptions of the “best” choice for public policy. We are not immune to this bias. Moreover, we do not deny that we had preconceptions, derived from our personal histories, our reading, and the hearings we held in 1996 to review Canada’s drug legislation. Among these preconceptions, which oriented our reading of the testimony, at least at first, we note:

··               The conviction that the current system does not achieve its objectives, if only because of the increase in cannabis use, by young people in particular;

··               A preference for an approach that is more consensual and more in keeping with Canadian attitudes;

··               A preference for a harm-reduction approach as indicated by the wording of our first mandate;

··               A tendency to distinguish between soft drugs – including cannabis – and hard drugs (heroin, cocaine);

··               A certain lack of knowledge about the specific effects of cannabis, from the standpoint of the toxicological and pharmacological studies conducted in recent years.

This being said, we did not work in isolation. Not only were we accompanied by our research team – sociologists, lawyers, criminologists – throughout our work, not only were we also under the close surveillance of the witnesses in a way and of the public in a larger sense, but primarily, other committees, in other countries, have conducted similar reviews in recent years. Their work was a source of inspiration and knowledge and as well a benchmark against which to compare our own conclusions.



[1][7] Testimony of Michael J. Boyd, Chair of the Drug Abuse Committee and Deputy Chief of the Toronto Police Service, for the Canadian Association of Chiefs of Police, Special Senate Committee on Illicit Drugs, First Session of the Thirty-Seventh Parliament, Issue No. 14, page: 74.

[2][8] Testimony of Dale Orban, Detective Sergeant, Regina Police Service, for the Canadian Police Association, Special Senate Committee on Illicit Drugs, First Session of the Thirty-Seventh Parliament, Monday, May 28, 2001, Issue 3, page: 47. It should be immediately noted that the last statement is completely false as we will see in Chapters 19 and 12 on international agreements and Canadian legislation that have placed cannabis on the list of controlled drugs since 1924, with no knowledge of its physical or psychological effects at that time, and for completely different reasons, when there were any.

[3][9]  Dr. Mark Zoccolillo, Professor of Psychiatry and Assistant Professor of Pediatrics, McGill University and the Montreal Children's Hospital, Special Senate Committee on Illicit Drugs, Second Session of the Thirty-Sixth Parliament, October 16, 2000, Issue No. 1, page 77.

[4][10]  Dr. Bill Campbell, President, Canadian Society of Addiction Medicine, Special Senate Committee on Illicit Drugs, First Session, Thirty-Seventh Parliament, March 11, 2002, Issue No. 14, page: 56.

[5][11]  Mr. John Conroy, Barrister, Special Senate Committee on Illicit Drugs, First Session of the Thirty-Seventh Parliament, March 11, 2002, Issue No. 14, page 11.

[6][12]  Dr Colin Mangham, Director, Prevention Source BC., Special Senate Committee on Illicit Drugs, First Session of the Thirty-Seventh Parliament, September 17, 2001, Issue No. 6, page: 71.

[7][13]  Dr. Benedikt Fischer, Professor, Department of Public Health Sciences, University of Toronto, Special Senate Committee on Illicit Drugs, First Session of the Thirty-Seventh Parliament, September 7, 2001, Issue No. 6, page 9.

[8][14]  Dr. Perry Kendall, Health Officer for the Province of British Columbia, Special Senate Committee on Illicit Drugs, First Session of the Thirty-Seventh Parliament, September 17, 2001, Issue No. 6, pages 33-33.

[9][15]  Dr. Harold Kalant, Professor Emeritus at the University of Toronto, Special Senate Committee on Illicit Drugs, First Session of the Thirty-Seventh Parliament, June 11, 2001, Issue No. 4, pages 74-76.

[10][16]  Dr. Mohamed ben Amar, Professor of Pharmacology and Toxicology, University of Montreal, Special Senate Committee on Illicit Drugs, First Session of the Thirty-Seventh Parliament, June 11, 2001, Issue No. 4, pages 9-10.

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