Dr. Harold Kalant is a professor emeritus of the University of Toronto’s department of Pharmacology and toxicology.  He was director of biological and behavioural research at the Addiction Research Foundation of Ontario (now the Centre for Addiction and Mental Health). Since 1959 Kr. Kalant’s research has been largely on alcohol and cannabis.

Following is Dr. Kalant’s presentation to the House of Commons Health Committee in May of 2014.

I thank the chair and members of the Standing Committee on Health for their invitation to present a few facts and interpretations on the topic of marijuana and health. My remarks will give special reference to young users.

Marijuana is not the most dangerous of drugs. There are no proven deaths attributable to overdose, and millions of people have used it in small doses and on infrequent occasions with no obvious adverse effects. However, this has given rise to a widespread but erroneous belief that marijuana is safe or harmless.

There is no such thing as a harmless drug. Everything with pharmacological action has the ability to produce harm, depending on the amount used, how often, for how long, by whom, and under what circumstances. Not surprisingly, the harmful effects of marijuana are most often found in heavier users and those with greater vulnerability.

Among those who begin to use marijuana as adults, the most common adverse effects include chronic inflammatory changes in the respiratory system, poor memory, poor work performance in activities requiring mental and physical skills, driving accidents, and addiction. The physical and mental effects usually recover on cessation of use.

However, adolescents and young adult users of marijuana greatly outnumber mature adult users. Young beginners, those who begin use as early as 12 or 13 years, are much more vulnerable to harmful effects and are, therefore, at greater risk. In those with a family history indicative of a genetic risk for schizophrenia, cannabis can precipitate overt clinical psychosis, and in those who have been treated, it increases the risk of relapse and of a difficult clinical course, with poorer results of treatment.

Less dramatic but of much wider application is the fact that the developing brain is more susceptible to the deleterious effects of cannabis. It has an inhibitory effect on the development of connections between parts of the brain involved in higher mental functions. This has been demonstrated in animal studies by histological examination of brain tissue, and in humans by brain imaging studies, showing thinning of relevant areas of the brain cortex in affected users and differences in regional blood flow and electrical activity of such areas. These alterations give rise to problems in such functions as memory, learning, reasoning, and problem solving, which are collectively referred to as “executive functions”.

Our laboratory was the first to show that animals treated before puberty, before reproductive age, with marijuana extract for a month, and then left without treatment for three months to allow complete elimination of the drug, showed long-lasting, apparently permanent impairments to learning. In the laboratory rat, I would point out, three months without the drug is equivalent to about nine years in humans, as a fraction of life expectancy. So these animals were indeed, long-term, free of the drug itself but still showed residual mental effects. Other groups have subsequently confirmed and extended these findings.

Two of the most striking demonstrations of long-lasting effects in humans have come from the Ottawa Prenatal Prospective Study and the Dunedin, New Zealand, birth cohort study.

The OPPS, the Ottawa study, followed groups of offspring of mothers who smoked cannabis during pregnancy in comparison with those of mothers who smoked tobacco or did not smoke at all. Those born of mothers who smoked cannabis showed only minor physical effects at birth that recovered fully during the first post-natal year, but when they reached school age, they showed mental effects that persisted throughout their growth and development and into their adult years. These were minor changes but sufficient to affect the educational attainments of the children.

The Dunedin study followed 1,037 individuals born in Dunedin during 1972-73, with repeated interviews and examinations at intervals of two or three years throughout childhood, before any of the children had started using cannabis, and again at intervals, after they had begun—that is, those who had begun—up to the age of 38 years, most recently.

Those who never acquired a habit of using cannabis showed a small increase in intelligence quotient at 38 years, compared to 13 years, but those who began to use it regularly showed losses in intellectual function that were greater the greater the amount of their use and the earlier the age at which they started.

These losses affected at least five different areas of mental functioning, and were shown not to be due to residual cannabis in the body, not to fewer years of schooling and not to pre-existing mental problems before cannabis use began. As well, they were largely clustered among those who had started use at the youngest ages. Those who began at later ages and ceased using cannabis recovered fully, but in those who started when youngest, cessation of use was not followed by full recovery. These mental effects resulted in more school dropout, poorer social adjustment, and a greater risk of depression later.

Adolescents using cannabis have also been involved in a growing number of motor vehicle accidents as drivers. Culpability analyses point to the cannabis-using drivers as the ones responsible for the accidents, and of course this is obviously the case in single-vehicle accidents. This appears to be due in part to their belief that cannabis does not impair their driving ability, and in part to their knowledge that there’s no analytical test for cannabis comparable to roadside breath tests for alcohol, so they feel they are less likely to be detected.

Finally, the risk of addiction is greater in young users of cannabis than in older ones. One study found that whereas the risk of addiction in regular users in general is about 10%, among adolescent regular users it is about 16%.

In conclusion, the use of cannabis for pleasure comes at a cost, and society must ponder whether the pleasure is worth the cost. Sound policy should be based on thorough, comprehensive, and well-researched cost-benefit analyses. The use of criminal sanctions against individuals possessing small amounts for personal use in my view does not benefit society, but society as a whole must give careful thought to changes in policy that could increase the number and severity of health problems caused by use by its more vulnerable members, which, as I have pointed out, means its younger users.

I would like to add, in response to the questions that a number of people asked earlier in this session, that the cost-benefit analysis of medical use is a quite different matter from the cost-benefit use of non-medical use. In medical use you balance the improvement in health and the importance of that improvement in health relative to the harms that may be caused by the dosage used for health purposes, which, as Dr. Kahan has pointed out, should typically be considerably less than the doses employed by regular users for non-medical purposes.

In contrast, for non-medical use, the benefits are not improvements in health and social function but temporary benefits in how one feels. If one is euphoric, one likes it and wants to do more. That’s fine, but then the question is whether that pleasure is worth the costs of the damage caused by the heavier doses that non-medical users tend to employ.