Minimum legal age for cannabis use should be 19, study suggests

A team of researchers at the Memorial University of Newfoundland, Canada, investigated how Canadians who started using cannabis at several young ages differed across important outcomes (educational attainment, cigarette smoking, self-reported general and mental health) in later-life.

Dr Hai Nguyen, lead author of the study said: "Prior to legalisation, the medical community recommended a minimum legal age of 21 or 25 for non-medical cannabis use in Canada. This recommendation was based on scientific evidence around the potential adverse impacts of cannabis on cognitive development. However, policymakers feared a high minimum legal age may lead to large underground markets, with those under the legal age continuing to use cannabis illegally. Ultimately, a lower legal age of 18 or 19 was decided across provinces, however there remains ongoing debate and calls to raise the legal age to 21."

To determine whether the age at which people begin to use cannabis may have an impact on later-life outcomes, the authors analysed data from nationally-representative Canadian Tobacco Use Monitoring Surveys (CTUMS) and Canadian Tobacco, Alcohol and Drugs Surveys (CTADS) conducted between 2004 and 2015, which annually interview up to 20,000 individuals aged 15 years and older.

The authors found different optimal minimum legal ages depending on the outcome of interest. For smoking, respondents who first used cannabis aged 19-20 were less likely to smoke cigarettes later in life than those who first used cannabis aged 18, but no significant difference was found in those who started using cannabis at an older age, indicating an optimum legal age of 19.

The number of respondents reporting a high level of completed education was 16% higher among those who first used cannabis between the ages of 21 and 24, relative to those who first used it before age 18, suggesting an optimal minimum legal age of 21.

General health was significantly better among those who started using cannabis aged 18, relative to those who started before age 18, but no significant difference was found among those who started at an older age, suggesting a minimum legal age of 18. However, mental health outcomes were found to be higher among those who first used cannabis aged 19-20 than before age 18, suggesting a minimum legal age of 19.

Dr Nguyen said: "The lower level of completed education reported in those who first used cannabis at an earlier age may reflect poor neurological development or a higher 'drop-out' rate from further education. It is also possible that those who initiate cannabis use early may use it as a gateway for further illicit drug use, resulting in poorer health in later life, which may explain the poor general or mental health scores recorded in the study."

Dr Nguyen said: "Taking into account all measured outcomes, our results indicate that, contrary to the Canadian federal government's recommendation of 18 and the medical community's support for 21 or 25, 19 is the optimal minimum legal age for non-medical cannabis use. Keeping the legal age below 21 may strike a balance between potential increases in underground markets and illegal use, and avoiding the adverse outcomes associated with starting to use cannabis at an earlier age."

The authors caution that as the study used self-reported data, respondents may not have accurately recalled the age at which they first used cannabis. Data was also collected prior to legalization of non-medical cannabis in Canada and the authors were unable to predict the impact of cannabis use after legalization. They suggest that further research is needed to establish potential causal effects between the age at which cannabis is first used and the outcomes measured in the study, and could also focus on additional outcomes such as driving behaviours and street drug use.