Source: Mar Fernandez/Shutterstock

The scientific literature on cannabis use is filled with examples of researchers developing more powerful tools and experimental protocols that find more and more subtle impacts from frequent cannabis use. Nadia Solowij, for example, first documented learning and memory difficulties in long-term heavy cannabis users. Then, upon improving her research methods, she was able to measure decrements in learning and memory in adolescents from as little as weekly use.

The logical conclusion to draw from this pattern is that there is probably not a threshold beyond which cannabis begins to have an impact. Rather, there is a continuum of impact beginning with any amount of use. While this sounds at first glance like a drastic and overwrought conclusion, it may be both true and easily managed. For example, while the number of our brain’s cannabinoid receptors begins to be reduced with a single episode of cannabis use, this reduction is quite temporary. If use is not repeated too quickly, the number of cannabinoid receptors quickly rebounds back to normal. This is fortunate, since good mental and emotional health depends on a well-balanced endogenous cannabinoid system with a normal complement of cannabinoid receptors. It is only when cannabis use is repeated too quickly and too often that the reduction in receptors becomes cumulative, and can reach 20 to 60 percent in different areas of the brain (see How Cannabis Defeats Itself When Used Too Frequently).

New research on the association between cannabis use and adverse psychosocial events now also contributes to the idea that cannabis’s impact exists on a continuum rather than only after reaching a threshold.

Ryan Sultan, et. al., recently reported a stepwise progression in adverse psychosocial events among cannabis non-users, non-dependent users, and users who meet criteria for Cannabis Use Disorder (CUD).[1] This cross-sectional study used data from face-to-face interviews with a nationally representative sample of adolescents aged 12 to 17 conducted from 2015 to 2019 by the National Survey on Drug Use and Health. Over 68,000 participants fell into three groups: 87 percent reported no cannabis use in the previous year, 10 percent had used during the previous year but did not meet DSM-5 criteria for Cannabis Use Disorder, and 2.5 percent met three or more DSM-5 criteria for Cannabis Use Disorder. The three groups were labeled non-users, non-disordered cannabis users (NDCU) which I choose to call moderate users, and Cannabis Use Disordered.

The next step in Sultan’s research was to compare the prevalence of a set of nine adverse events suffered by each of the three groups. The nine adverse events included major depression, suicidal ideation, slower thoughts, difficulty concentrating, truancy, low grade point average, arrest, fighting, and aggression. Prevalence of the various adverse events for adolescents with CUD ranged from 12.6 percent to 41.9 percent, while the adverse events ranged from 5.2 percent to 30.4 percent for moderate cannabis users and from 0.8 percent to 17.3 percent for non-users. Compared with non-users, moderate cannabis users had approximately two to four times greater odds of all nine adverse psychosocial associated events.

The study has one major limitation. It can’t say anything about cannabis causing the greater prevalence of adverse events. Perhaps there are underlying factors that independently lead to both an increased prevalence in cannabis use and an increased prevalence of negative events. Three comments occur to me regarding this limitation.

First, even if the relationship between cannabis use and adverse events is only an association and not causation, moderate cannabis use can still be used as a clinical marker to identify individuals at increased risk of adverse events, whether because of cannabis or not. For this reason alone, Sultan’s study provides valuable clinical information.

Second, other studies support the possibility cannabis may play a causative role in producing adverse consequences. Animal studies consistently find structural changes caused by THC in areas of the brain that subserve cognitive and emotional functions related to the adverse events identified by Sultan. In addition, early onset of cannabis use is associated with greater likelihood of negative consequences.

And third, I agree with the study authors that the criteria used by DSM-5 may be more appropriate for diagnosing adults with problematic cannabis use than for adolescents. This may be true particularly because adolescents’ still developing brains encounter potential adverse impacts from cannabis not experienced by adults.