The Scary, Mostly Hidden Truth About Cannabis and Mental Illness

By Eric D. Collins, MD
Dr. Collins, the Medical Director at the New York Center for Living, has practiced addiction psychiatry for nearly 30 years.

Over the last approximately 25 years, the medicalization and legalization of marijuana throughout much of the United States have promoted the myth that marijuana is not a dangerous drug. An accumulating body of research, including a recent analysis of fifty years of population data from Denmark, suggests that contrary to the myth, cannabis use disorder (CUD) can cause schizophrenia—a devastating, chronic mental illness—and this cannabis-schizophrenia connection disproportionately affects young males.

In the last 15-to-20 years, there has been a growing recognition that cannabis use can cause not only psychotic symptoms, but schizophrenia. In 2007, the editors of a major medical journal, The Lancet, captured this concern when they wrote the following:

In 1995, we began a Lancet editorial with the since much quoted words: “The smoking of cannabis, even long term, is not harmful to health.” Research published since 1995, including Moore’s systematic review in this issue, leads us now to conclude that cannabis use could increase the risk of psychotic illness.  … [G]overnments would do well to invest in sustained and effective education campaigns on the risks to health of taking cannabis.” (Editors of The Lancet, 7/28/07)

Unfortunately, it appears that governments, including the U.S. government, have not generally followed the advice suggested by The Lancet editors. Since 1995, the percentage of THC in marijuana seized by the DEA has increased from about 4% (1995) to 15% (2021). The percentage of THC in some specialized, highly-concentrated products provided by dispensaries can be over 90%. And there is evidence that the increase in THC potency (percentage of THC in marijuana) increases the risk of developing CUD. Notably, the percentage of all schizophrenia thought to be caused by CUD has increased three-to-four-fold in the last 20 years.

In May 2023, researchers published an analysis of national health data from Denmark, covering the period from 1972 to 2021. It showed a significant increased risk of schizophrenia among people with CUD compared to those without CUD. Moreover, the association was much stronger for males than females—particularly young males. The authors concluded that roughly 15% of cases of schizophrenia among males would have been prevented in the absence of CUD, in contrast to 4% among females. They also estimated that among younger males, the proportion of preventable CUD-associated cases might be as high as 25% or 30%. They speculated that the increasing percentage of schizophrenia cases caused by CUD was likely related to increasing potency of cannabis and higher rates of CUD over time.

Where does this leave us? It leaves us with a lot of work to do to correct the misinformation that cannabis is safe—especially the myth that it is safe for adolescents. While its risks are clearly different from those of other drugs, including alcohol, cannabis clearly carries significant risks, including that it can cause both addiction and worsened mental illness. A more recent study, also published in May 2023, showed that even more modest adolescent cannabis that does not rise to the level of a cannabis use disorder is associated with increased risks of major depression, suicidal ideation, slower thoughts, difficulty concentrating, truancy, low grade point average, arrest, fighting, and aggression. Taken together, these two recent studies add to a growing and compelling body of research demonstrating a connection between adolescent cannabis use and mental illness, including the devastating and typically lifelong mental illness, schizophrenia.

Given the old adage, an ounce of prevention is worth a pound of cure, the medical evidence clearly points to the need for focused efforts to prevent adolescents and young adults from using cannabis until approximately age 26, when the mental health risks described here seem to abate. Prevention starts at home, and I urge all parents of adolescents and young adults to make it clear to their children that use of cannabis and all manufactured THC-containing products is seriously risky to mental health and to take steps to prevent cannabis use in their children. I also urge adoption of a national campaign to get the word out that cannabis use is particularly dangerous to young people. This campaign should include mandatory warning labels on all cannabis products, similar to those that appear on tobacco products and alcohol. Beyond this, our country needs to make mental health care much more readily available, as ballooning rates of mental illness among young people likely contribute to a tendency for those young people to self-medicate with cannabis. Only with concerted individual, community, and national efforts such as these and others can we reliably mitigate the very serious and mostly unexamined risks posed by widespread legalization and medicalization of cannabis.