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A recent study in The Lancet has found that high frequency use of high potency cannabis can increase risk of psychosis. The risk doubles for some teenagers.

Frequent use of high potency cannabis can increase odds of developing psychosis. The effect doubles in some teenage users. The recent legalization of cannabis in Canada has been met with both appraisal and criticism. Enthusiasts call it revolutionary for medicine and the judicial system, whereas skeptics warn of the possible dangers associated with its use.

Recently, Israel decriminalized marijuana nationwide. This once again probed the ongoing debate of whether cannabis is safe or not. Typically, cannabis is regarded as much safer than alcohol by enthusiasts. Afterall, alcohol consumption increases the risk of esophageal and mouth cancer in all genders, and it increases the risk for breast cancer in women (Corrao et al., 2004). Further, it is closely linked to poor-decision making. According to the World Health Organization (WHO, 2018), alcohol accounts for 1 in 20 deaths worldwide each year. Bare in mind that this list is not exhaustive.

When in juxtaposition with alcohol, cannabis does appear ‘safer.’ However, that is untrue. Sort of. Indeed, some of the problems associated with alcohol use are unique to the substance but, cannabis is not without its own risks. Like alcohol, the context in which cannabis is consumed is relevant. Responsible, recreational, and occasional use of either drug is associated with significantly less harm. There may be an exception to this for cannabis in medicinal contexts where it is used to substitute more addictive drugs like opioids to treat pain. However, when misused, both drugs pose a serious threat.

A recent study in The Lancet (Forti et al., 2019) added to the current body of evidence that there is a strong effect in daily cannabis use of high potency cannabis and the odds for developing a psychotic disorder. In other words, the more frequent you smoke cannabis and the higher the potency, the more likely you are to develop a psychotic disorder. This is the largest study of this kind to date.

This study is unique due to its attempt to control for extraneous and confounding variables. For example, the researchers considered the possibility of a synergistic effect between cannabis and other substances (e.g. alcohol) leading to psychosis. Their analysis allowed them to say confidently that it is indeed cannabis alone and the nature of the dose and the frequency that lead to psychotic disorders. In fact, they found that alcohol was not a mediating factor in this effect. In other words, individuals still developed psychoses regardless of whether they consumed low, medium, or high amounts of alcohol in addition to their cannabis use.

In addition to alcohol, Forti and colleagues explored whether age was a mediating factor in this effect. As it happens, it is. Participants in the study who began using high potency cannabis by the age of 15 were twice as much at risk for developing psychoses. This is especially relevant to policy makers. Legalization of cannabis does reduce the black market and allows for tighter control of the substance; reducing the possibility of teenagers gaining access to the substance. However, it is still important to note that regardless of age, higher doses and frequency of cannabis use still puts individuals at risk for developing psychosis. In fact, this combination accounted for one fifth of all new cases of psychosis in the study. Therefore, the researchers concluded that 20% of all new cases of psychosis could have been prevented by removing daily cannabis use. Legal, adult cannabis users ought to be aware of the risk.

As research continues, more data will be available. Until recently, the risks associated with cannabis use were mostly speculatory based on limited data. However, legalization has curtailed this barrier. The evidence published by Forti and colleagues in the Lancet is extensive and supports previous findings on the matter. Although, it is still not possible to conclude that there is a direct causal effect between cannabis use and the development of psychosis in humans, the current body of evidence does suggest a strong correlation between the two. Enough to merit caution. It is important to note that studies involving rats have been able to conclude a causal relationship between the two (Szkudlarek et al., 2018) and typically, rats are strong predictive models for humans.

The purpose of this article is to shed light on the risks associated with cannabis use -- not to deter individuals from using it. It is important to remain informed. Like any substance, moderation and responsible use can mitigate some of the risks. However, not negate them. Further, this evidence does not necessarily apply to those who require cannabis for medicinal purposes. There is a large body of literature that supports the efficacy of cannabis in chronic pain, especially that of which is related to cancer (Romero-Sandoval, Kolano, & Alvarado-Vazquez, 2017).

As part of an initiative to mitigate risks, the Province of Nova Scotia developed resources for responsible cannabis use. These recommendations are evidence-informed and up-to-date. If you or a loved one are struggling to use cannabis responsibly or would like more information, please consider speaking to one of our counsellors today.

Other useful resources

Cannabis in Canada

Sobriety Home

Cannabis | Canadian Centre on Substance Abuse and Addiction


Corrao, G. (2004). A meta-analysis of alcohol consumption and the risk of 15 diseases. Preventive Medicine, 38(5), 613–619. doi:10.1016/j.ypmed.2003.11.027

Forti et al., (2019). The contribution of cannabis use to variation in the incidence of psychotic disorder across Europe (EU-GEI): a multicentre case-control study. Lancet Psychiatry. 2019 Mar 19. pii: S2215-0366(19)30048-3. doi: 10.1016/S2215-0366(19)30048-3.

Romero-Sandoval EA, Kolano AL, Alvarado-Vázquez PA. Cannabis and Cannabinoids for Chronic Pain. Curr Rheumatol Rep. 2017 Oct 5;19(11):67. doi: 10.1007/s11926-017-0693-1. Review. PubMed PMID: 28983880

Szkudlarek HJ, Desai SJ, Renard J, Pereira B, Norris C, Jobson CEL, Rajakumar N, Allman BL, Laviolette SR. Δ-9-Tetrahydrocannabinol and Cannabidiol produce dissociable effects on prefrontal cortical executive function and regulation of affective behaviors. Neuropsychopharmacology. 2019 Mar;44(4):817-825. doi: 10.1038/s41386-018-0282-7. Epub 2018 Nov 27. PubMed PMID: 30538288.

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