How addictive is heroin / morphine?

The only real answer to this question is "it depends." That is, it depends on what you mean by "addiction" and the individual and the environment in which they find themselves.

For example, there are some people who are said to be "addicted" to gambling. The problem is that gambling has no physical substance. If people can become "addicted" to something with no physical substance, then we should ask ourselves what "addiction" really means, and how "addictive" a physical substance is when things with no substance can cause "addiction."

In an attempt to define clearly what is meant by addiction, and which drugs are the most addictive, Dr. Jack E. Henningfield of the National Institute on Drug Abuse and Dr. Neal L. Benowitz of the University of California at San Francisco ranked six psychoactive substances on five criteria.

• Withdrawal -- The severity of withdrawal symptoms produced by stopping the use of the drug.
• Reinforcement -- The drug's tendency to induce users to take it again and again.
• Tolerance -- The user's need to have ever-increasing doses to get the same effect.
• Dependence -- The difficulty in quitting, or staying off the drug, the number of users who eventually become dependent
• Intoxication -- The degree of intoxication produced by the drug in typical use.

The tables listed below show the rankings given for each of the drugs. Overall, their evaluations for the drugs are very consistent. It is notable that marijuana ranks below caffeine in most addictive criteria, while alcohol and tobacco are near the top of the scale in many areas.

The rating scale is from 1 to 6. 1 denotes the drug with the strongest addictive tendencies, while 6 denotes the drug with the least addictive tendencies.

HENNINGFIELD RATINGS

Substance   Withdrawal   Reinforcement   Tolerance   Dependence  Intoxication
Nicotine         3             4             2           1            5

Heroin           2             2             1           2            2

Cocaine          4             1             4           3            3

Alcohol          1             3             3           4            1

Caffeine         5             6             5           5            6

Marijuana        6             5             6           6            4

BENOWITZ RATINGS

Substance   Withdrawal   Reinforcement   Tolerance   Dependence  Intoxication

Nicotine        3*             4             4           1            6

Heroin           2             2             2           2            2

Cocaine         3*             1             1           3            3

Alcohol          1             3             4           4            1

Caffeine         4             5             3           5            5

Marijuana        5             6             5           6            4

*equal ratings

While these rankings may be helpful in comparing overall "addictive" qualities for millions of people, it should be noted that these rankings don't describe the reactions of any one individual. Whatever the rankings may be, people will have their own personal preferences and reactions and, it must also be said, most people will never have a serious problem with addiction to any of these drugs.

For example, while heroin may get numbers near the top, research has shown that most people don't really like heroin when they try it, and only a small minority like it well enough that they will continue using it. For the small minority that really like it, it may be hugely addictive. For the majority of us, however, it will have little to no appeal as a recreational drug. You can perhaps see this if you look around you at the people you know who drink alcohol. Some people drink it in copious quantities, and apparently love it. For other people, one beer will give them a headache and a hangover the next day. 

The bottom line is that "addiction" is more the product of internal factors in the individual than it is the product of a particular drug. If you want to see some further proof of this, watch what happens when someone stops abusing a particular drug. In all probability, they will replace the habit with something else.