In 1987, a research team published “Cannabis and Schizophrenia: A Longitudinal Study of Swedish Conscripts” in The Lancet journal that suggested “cannabis is an independent risk factor for schizophrenia.” More research followed, including a 2004 British Journal of Psychiatry study that said ending all cannabis use would decrease incidence by eight percent worldwide. The findings also suggest that adolescent use in particular increases risk. Support for this myth, however, is not holding up.
First, consider the anecdotal evidence. In 1969, the Gallup poll first started asking questions about illegal drugs, and only four percent of American adults in that survey had ever tried cannabis. Alana Anderson, a college graduate quoted in that 1969 poll, said, “My generation was told that marijuana caused acne, blindness and sterility.” The percentage of people who tried cannabis grew to 24 percent by 1977 and 33 percent by 1985. In the 2015 Gallup poll, 44 percent admitted to trying cannabis, and 58 percent wanted to see it legalized.
The number of people who tried cannabis increased elevenfold between 1969 and 2015, during which time cannabis potency also increased significantly. If cannabis increases the risk for developing schizophrenia, surely the rate of incidence increased proportionally to the rise in cannabis consumption. In reality, schizophrenia rates remained steady (1.1 percent of American adults), and the 1995 Schizophrenia Bulletin study “Time Trends in Schizophrenia” said rates actually seem to be in decline over the past few decades.
Now for the clinical evidence. Several recent studies have not found a cannabis-schizophrenia connection, and more importantly, they found no connection between adolescent use and schizophrenia. For example, a 2005 study in Biological Psychiatry found that “cannabis use had no such adverse influence” and that “the vast majority of young people who use cannabis do not develop psychosis.” The 2015 study “Chronic Adolescent Marijuana Use as a Risk Factor for Physical and Mental Health Problems in Young Adult Men” in Psychology of Addictive Behaviors also found no connection. Lead researcher Jordan Bechtold explained in a press release, “There were no differences in any of the mental or physical health outcomes that we measured regardless of the amount or frequency of marijuana used during adolescence.”
In 2011, Archives of General Psychiatry published “Cannabis Use and Earlier Onset of Psychosis” that argued in favor of the cannabis-schizophrenia connection noting that symptoms typically manifest 2.7 years earlier for those who consume cannabis. Still, the authors admitted, “Not all researchers agree that the association between cannabis use and earlier age at onset is causal.” Some studies “argue that the association between cannabis use and earlier age at onset could be explained by demographic variables, including lower socioeconomic status and the proportion of male cannabis users” and that “the apparent association between earlier age at onset and cannabis use might simply be owing to older patients with first-episode psychosis being less likely to use cannabis.”
If the basis for this argument is that symptoms manifest 2.7 years earlier in people who consume cannabis, there might be an even simpler explanation. As any addiction therapist can attest, mental health disorders are leading drivers for substance abuse, and many people turn to illicit and prescription drugs to self-medicate symptoms. During the early stages of schizophrenia, many people might turn to alcohol, cannabis or prescription drugs to mute the symptoms. In other words, schizophrenia is the risk factor for cannabis use, not the other way around.