Professor Ruth Ross is Chair of the Dept of Pharmacology and Toxicology at the University of Toronto and a Senior Scientist at CAMH. Professor Harold Kalant is Professor Emeritus in the Department of Pharmacology and Toxicology at the University of Toronto and Research Director Emeritus (Biobehavioral Studies) at CAMH. Both have many years of research experience in the field of cannabis research.

The recent Canadian legislation to legalize cannabis was purportedly based on public health considerations, including the need to keep marijuana out of the hands of children and adolescents. This was amply justified by the known special vulnerability of the young. Adolescents and young adults make up about half of all cannabis users in Canada, and the earlier in adolescence they start using cannabis, and the longer and more heavily they use it in the period of brain maturation, the greater is the risk of apparently permanent impairment of higher brain functions. CAMH “Facts and Statistics” pages indicate that young Canadians “aged 15-24 are more likely to experience mental illness and/or substance use disorders than any other age group”, and suicide is among their leading causes of death. Cannabis can also increase the likelihood that an adolescent with hereditary susceptibility to schizophrenia will actually develop an active psychosis.

But health problems due to cannabis use are not limited to adolescents. The human brain does not reach full maturity until at least the mid-20s. Young adult users are also vulnerable to adverse effects such as accidents, psychosis, addiction, and impaired post-secondary education and work performance. The Cannabis Act set 18 years as the minimum age for legal purchase and possession of cannabis but allowed the provinces to raise this if they so decided. So far, no province has changed it to more than 19. Therefore, young adults have been ignored as an age group also in need of public health protection.

Most surprising, perhaps, is the recent recognition that the elderly are also becoming an unanticipated target of cannabis-related health problems. US national survey data indicate that though cannabis use is greatest among 25-35-year-olds in absolute numbers, the percentage increase has been greatest among the 65+ age group – a 250% increase from 2006 to 20131. Increasing numbers of the elderly are turning to cannabis for intended medical purposes, and thereby becoming subject to the adverse interactions between cannabis and common effects of aging2,3. This can give rise to an increased risk of falls, impaired mobility, cognitive deterioration, hypotension and other health problems that are most common among the elderly. Physicians are only starting to recognize the risk of cannabis use disorder in the elderly and to realize the difficulty of recognizing it because of the overlap between its symptoms and those of aging itself.

Thus, all age groups deserve the government’s attention to its promise to control the use of legalized cannabis in accord with best public health principles. Yet it is becoming glaringly obvious that the public media are far more interested in the prospects of huge financial gains from the sale of “recreational” cannabis, and in the normalization of cannabis use in many aspects of daily life. This focus is most likely to increase, rather than control, the total extent of use among all age groups, and experience with alcohol, tobacco, and other drugs has shown that increased total use is quite predictably accompanied by increased harm. The message of “taking cannabis sales out of the hands of criminals” has been overemphasized at the expense of the parallel public health imperative of reducing problematic cannabis use rather than simply changing the source or, worse, increasing the “market”. History should alert us to the dangers of profit-driven sale and marketing of substances that can cause harm. The causal link of cigarette smoking with lung cancer was confirmed by strong evidence in the 1950s. Yet it took many years more for this to be widely accepted, in part due to denial of the evidence by cigarette manufacturers. One of their most insidious tactics was to strategically create scientific controversy which, in turn, seriously undermined efforts to provide a robust public health message4. Contentions of scientific uncertainty or “lack of evidence” remove the onus of corporate or government responsibility for harms. It may similarly take society many years to recognize the actual extent of health problems resulting from increased use of cannabis. What we don’t know can hurt us.

If the government really wishes to adhere to public health principles rather than to a profit motive for cannabis legalization, it is therefore essential to implement as rapidly as possible a series of measures to monitor the consequences of cannabis legalization and recognize problems as early as possible, so that they can be tackled before they become too strongly established. One is to begin immediately recording present levels of use and associated health problems by regional, age and sex groupings, as a baseline against which to compare future levels in regular post-legalization surveys5. Another is to develop and launch, as early as possible, age-appropriate educational programs6 to make clear to the public the potential harms of premature and excessive use, highlighting risks to specific vulnerable groups. A third is to disavow publicly the temptation to view cannabis sale as an important source of revenue, and emphasize that, like alcohol and tobacco, it is capable of generating social and economic costs that may outweigh the financial gains7. Failure to do these will make a mockery of their declared commitment to public health goals.

1 Han BH, Sherman S, Mauro PM, Martins SS, Rotenberg J, Palamar JJ (2017). Demographic trends among older cannabis users in the United States, 2006–13. Addiction 112:516-525.
2 Conn D, Bertram J, Teed R, Fernando I (2018). Marijuana and older adults: So many burning questions. CSPC editorials, 20 August 2018. http://sciencepolicy.ca/news/marijuana-and-older-adults-so-many-burning-…
3 Royal College of Psychiatrists (2015). Substance misuse in older people: an information guide. https://www.rcpsych.ac.uk/pdf/Substance-misuse-in-Older-People
4 Brandt AM (2012). Inventing conflicts of interest: a history of tobacco industry tactics. American Journal of Public Health 102: 63–71.
5 Fischer B, Russell C, Rehm J, Leece P (2018). Assessing the public health impact of cannabis legalization in Canada: core outcome indicators towards an ‘index’ for monitoring and evaluation. Journal of Public Health pp. 1-10. doi:10.1093/pubmed/fdy090
6 Porath-Waller AJ, Beasley E, Beirness DJ (2010). A meta-analytic review of school-based prevention for cannabis use. Health Education & Behavior 37:709-723 doi: 10.1177/109019811036131 7 Canadian Substance Use Costs and Harms, 2007-2014 (2018). Canadian Centre on Substance Use and Addiction (CCSA), Ottawa, ISBN 978-1-77178-491-7.