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Reid K Hester Ph.D.
Reid K Hester Ph.D.

New Risks in Smoking Marijuana, Especially High-Potency Weed

New research links frequent use of marijuana to psychotic episodes.

A study published recently in The Lancet Psychiatry has found significantly increased odds of first psychotic episodes in individuals who smoke marijuana daily and/or who smoke high potency (THC > 10%) cannabis. This was a well-designed study with multiple sites in Europe and Brazil and a sample size of 2,138 (901 patients with psychotic episodes and 1,237 population controls from the same sites).

Compared to controls who never used cannabis, those who used it daily had an adjusted odds ratio of 3.2 of having a psychotic episode. And combined daily use with high-potency THC increased the odds ratio to 4.8. Furthermore, the researchers concluded that if high-potency THC were not available, the number of first episode psychosis would have been 12% less across the 11 sites, 30% less in London and 50% less in Amsterdam. Wow. One should be careful though not to infer causality which the study could and did not do. In addition the two samples may have been different in other ways that could put those who experienced the psychotic episode at a higher risk in the first place (e.g., history of childhood trauma).

Clearly, the high-potency marijuana available today is a very different drug compared to what many in the 1960s and 1970s smoked. While there is some disagreement about this (in part because testing methods have become more sensitive), the general consensus is that THC levels have increased over the years.

These data have implications for individuals who use cannabis. If you smoke, vape, or eat cannabis every day, you could be at increased risk of having a psychotic episode. And using high-potency cannabis daily may be tantamount to playing with fire. Scales of how high a person is describing an increasing loss of touch with reality as people reach levels 9 and 10 on a 1-to-10 rating scale. The is the essence of a psychotic episode.

The flip side of this scary scenario is that if you’re going to use cannabis, you can reduce your risk of psychosis by a) not smoking daily, and more important, b) choosing less potent strains of THC. As some dispensaries in California advise, “start slow and low” (in THC content).

The data also have public health implications. While some states have limited the THC content of edibles, we may also want to have a science-based discussion about the concentration levels available in cannabis products.

I’m not trying to be an alarmist or to dredge up the “reefer madness” of years gone by. But at the same time, we need to respect the data, its limitations but also its implications.


Di Forte, M., Quatronne, D, 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. The Lancet Psychiatry, DOI:

About the Author
Reid K Hester Ph.D.

Reid K. Hester, Ph.D., is the Director of Research at CheckUp & Choices, a digital health company that helps reduce alcohol and drug misuse, and a professor of psychology at the University of New Mexico.

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