Key Takeaways:
Cannabis users can reduce the risk of harm by avoiding high-risk practices, most notably: use before early-mid twenties, use of high-THC product, combustion inhalation, daily use, and driving while impaired.

Population data tell us that about one in ten Canadian cannabis users experience cannabis use disorder, characterized by an inability to control one’s use of cannabis in the face of problems associated with its use1.

The clinically-relevant question that this finding provokes is why some cannabis users develop problems while the great majority do not. In most cases, this will be determined by the patterns and circumstances of the drug’s use. It is therefore useful to think of lower-risk and higher-risk practices. Accordingly, people can reduce their risk for harm by using drugs in a safer manner. Evidence-based lower-risk guidelines have existed for alcohol use for several years2. In the case of cannabis, a recent review of evidence for circumstances of harm, led by Fischer, has been published in The American Journal of Public Health3. The paper provides a summary of evidence on higher-risk use of cannabis, and translates the evidence into recommended guidelines for lower-risk use.

The authors make the point that the only way to ensure that one does not experience any harm from personal use of cannabis is to abstain altogether.

The authors also cite substantial evidence that cannabis use early in life is associated with dependence and harm to one’s health and social well-being in early adulthood. While the strongest evidence of harm is for use during teenage years, the elevated level of risk continues into mid-twenties. The Guidelines recommend delaying the use of cannabis well into early adulthood.

Elevated risk for behavioural and mental health problems arising from cannabis use appears to be associated with higher-THC content products. Users can reduce risk by limiting the amount of THC ingested (and the level of impairment experienced). The Guidelines recommend using lower-THC products, or those with a higher CBD:THC ratio.

While the evidence is acknowledged as limited, it appears that the use of synthetic cannabinoids may be associated with adverse health consequences, including serious ones, and even some deaths. The Guidelines recommend avoiding the use of synthetics until more is known about their effects and specific risks.

Smoking cannabis in the traditional manner that uses combustion carries many of the same well-known health risks as smoking tobacco. There are safer alternatives. Electronic vaporization devices (vaping) heat cannabis (or tobacco) to produce a vapour rather than combust it to produce smoke. Based upon current knowledge, vaping is considered a safer alternative. However, the authors cite research showing that health risks have also begun to arise with this relatively new technology. Another alternative is edible cannabis products. This avoids risk associated with inhalation but introduces other types of risk – mostly related to dosage control. This can be particularly risky for novice users who do not have the tolerance or experience for dealing with higher-than-expected levels of impairment. Given our current level of knowledge, the Guidelines recommend a cautious use of edibles as the safest approach. Combustion should be avoided.

Deep inhalation and breath-holding are techniques used to maximize dosage and effect from cannabis. These practices also increase exposure to any harmful constituents of cannabis as well as to any adulterants that may be present. Thus, there is additional risk for harm to one’s respiratory system and overall health. The Guidelines recommend avoiding these techniques. While that may be less efficient, it will yield some benefit in reducing longer-term adverse health impact.

Daily or near-daily use of cannabis is associated with a higher risk to health and for social harm. The clinically-significant point is to avoid use that becomes habitual to the extent that it interferes with one’s obligations, quality of life and aspirations. The Guidelines recommend limiting cannabis use to once per week or on weekends (non-work days) only.

There is substantial evidence that driving while impaired on cannabis is associated with increased motor-vehicle accident involvement. The risk is even more significant considering that impaired driving also places others at risk. It is reasonable to assume that the risk would also apply to the operation of any kind of conveyance or dangerous machinery. The Guidelines recommend avoiding these activities for at least six hours following use of cannabis, and recommend against the combined use of cannabis and alcohol on a single occasion when driving as this multiplies the level of impairment.

While the level of risk and the mechanisms are not understood, the authors invoke cautionary principles to advise against the use of cannabis by pregnant women to protect the developing fetus, and also by anyone with a predisposition for, or a first-degree family history of, psychosis or substance use disorders.

Finally an intuitive-level recommendation from the authors is for cannabis users to avoid a combination of any of the higher-risk practices as this would likely multiply the level of risk.

There is still much to learn about lower- and higher-risk practices of cannabis use. As the evidence quality improves, so too will the guidelines evolve.


Author Details

The latest scientific evidence on this topic was reviewed by the Centre's leadership team. This evidence brief is written by Michael DeVillaer, assessed for accuracy by Co-Director Dr. James MacKillop, PhD, an expert in addictions and mental health research. There are no conflicts of interest. Questions regarding this piece should be directed to Dr. James MacKillop (jmackill@mcmaster.ca).


References
  1. Fischer B, Imtiaz S, Rudzinski K, Rehm J. Crude estimates of cannabis-attributable mortality and morbidity in Canada – implications for public health focused intervention priorities. Journal of Public Health (Oxf) 2016; 38(1): 183-8. 
  2. Rehm J,  Room R, Taylor B. Method for moderation: Measuring lifetime risk of alcohol-attributable mortality as a basis for drinking guidelines. International Journal of Methods in Psychiatric Research 2008; 17(3): 141–151.
  3. Fischer B, Russell C, Sabioni P, van den Brink W, Le Foll B, Hall W, Rehm J, Room R. Lower-risk cannabis use guidelines: A comprehensive update of evidence and recommendations.  American Journal of Public Health 2017; 107(8): e1-e10.