Q: I heard there are a ton of studies that show a clear connection between cannabis use and developing schizophrenia and other psychoses. Is this true?

Thanks for your question! As recreational use of marijuana is decriminalized or legalized in a growing number of states, these are important concerns to address. During my graduate work – I received my Ph.D. in Neuroscience from Albert Einstein College of Medicine – I became really interested in how we could better use scientific data to inform policy decisions, so I really appreciate and am happy to tackle this question.

The short answer is yes – there is a well-established connection between schizophrenia and cannabis use. The long answer, of course, is a bit more nuanced. An established connection between the two does not necessarily mean that cannabis use causes schizophrenia in otherwise healthy individuals. In fact, the evidence supporting a causal relationship between cannabis and schizophrenia is rather ambiguous.

The link — Patients with schizophrenia use cannabis at higher rates than the general population.

While it has been shown that the active ingredient in cannabis, tetrahydrocannabinol (THC), can cause psychotic episodes¹Murray RM, Englund A, Abi-Dargham A, et al. Cannabis-associated psychosis: Neural substrate and clinical impact. Neuropharmacology. 2017;124:89-104. https://www.ncbi.nlm.nih.gov/pubmed/28634109, these cannabis-induced psychoses are transient and do not lead to mental illness.

The link between cannabis and long-term mental illness, in particular schizophrenia, originated with a study published in 1987 that examined drug use and psychiatric disorders in Swedish inductees to the military²Andreasson S, Allebeck P, Engstrom A, Rydberg U. Cannabis and schizophrenia. A longitudinal study of Swedish conscripts. Lancet. 1987;2(8574):1483-1486. https://www.ncbi.nlm.nih.gov/pubmed/2892048. The study found that, after controlling for other social and medical factors, cannabis use was an independent risk factor for the development of schizophrenia²Andreasson S, Allebeck P, Engstrom A, Rydberg U. Cannabis and schizophrenia. A longitudinal study of Swedish conscripts. Lancet. 1987;2(8574):1483-1486. https://www.ncbi.nlm.nih.gov/pubmed/2892048. In other words, individuals that reported using cannabis on 50 or more occasions were approximately six times as likely to be diagnosed with schizophrenia later in life. The finding that people who have been diagnosed with schizophrenia or first-episode psychosis report higher rates of previous or current cannabis use than the general population is perhaps one of the strongest and most consistently-reported pieces of evidence demonstrating, at the very least, a notable relationship between schizophrenia and cannabis³Ksir C, Hart CL. Cannabis and Psychosis: a Critical Overview of the Relationship. Curr Psychiatry Rep. 2016;18(2):12. https://www.ncbi.nlm.nih.gov/pubmed/26781550. However, as Andréasson et al (1987) state in the discussion of their findings, “A statistical association between cannabis consumption and schizophrenia does not necessarily imply a causal association.”²Andreasson S, Allebeck P, Engstrom A, Rydberg U. Cannabis and schizophrenia. A longitudinal study of Swedish conscripts. Lancet. 1987;2(8574):1483-1486. https://www.ncbi.nlm.nih.gov/pubmed/2892048 That’s right, correlation does not equal causation, and the evidence in support of the hypothesis that cannabis use causes schizophrenia in otherwise healthy individuals is not so clear.

Tyler Vigen, a Harvard Law School student and former military intelligence analyst, created a website illustrating some particularly humorous conclusions that can be drawn from data that is merely correlated. Some of these correlations have been collected in the book, Spurious Correlations.

Correlation does not equal causation — Most people that use cannabis do not have schizophrenia.

After all, of the approximately 19.8 million (about 14%) people in the US aged 12 or older in 2014 that used cannabis, most have not developed schizophrenia. The prevalence of schizophrenia and related psychotic disorders in the US is less than 1%⁴NIMH. Schizophrenia. https://www.nimh.nih.gov/health/statistics/schizophrenia.shtml. Accessed May 29, 2019. Moreover, while cannabis use spiked in the 1970s and has increased again recently⁵Johnston L, Miech, RA, O’Malley, PM, Bachman, JG, Schulenberg, JE, & Patrick, ME. 2019. http://monitoringthefuture.org/pubs/monographs/mtf-overview2018.pdf⁶NIDA. Nationwide Trends. https://www.drugabuse.gov/publications/drugfacts/nationwide-trends. Accessed May 29, 2019., the prevalence of schizophrenia has remained stable⁷McGrath J, Saha S, Chant D, Welham J. Schizophrenia: a concise overview of incidence, prevalence, and mortality. Epidemiol Rev. 2008;30:67-76. https://www.ncbi.nlm.nih.gov/pubmed/18480098⁸Barbato A. Schizophrenia and Public Health. 1996. Accessed May 29, 2019. https://apps.who.int/iris/bitstream/handle/10665/63837/WHO_MSA_NAM_97.6.pdf?sequence=1&isAllowed=y. While it is true that correlation does not equal causation, if cannabis use caused schizophrenia, one might expect some correlation between the incidence of schizophrenia and rates of cannabis use. In other words, if cannabis use caused schizophrenia, when cannabis use drastically increased in the 1970s, one would have expected the rate of individuals diagnosed with schizophrenia during that time period to have increased, as well.

So what accounts for the connection between cannabis and schizophrenia?

Cannabis use and schizophrenia may have shared genetic causes.

A plausible explanation for this phenomenon that also accounts for the low prevalence of schizophrenia, despite widespread cannabis use, is that they share a set of genetic causes. Power et al (2014) examined the association between genetic risk for schizophrenia and cannabis use in a large sample of healthy individuals. They found that individuals whose genomes contain a larger number of schizophrenia-related genes are both more likely to have ever used cannabis and to use it in greater quantities⁹Power RA, Verweij KJ, Zuhair M, et al. Genetic predisposition to schizophrenia associated with increased use of cannabis. Mol Psychiatry. 2014;19(11):1201-1204. https://www.ncbi.nlm.nih.gov/pubmed/24957864. I would like to highlight that the population sampled in this study are not patients with schizophrenia; thus, the conclusion that can be drawn is that healthy individuals with a genetic predisposition for schizophrenia consume more cannabis than the general population. It is possible that the genes that cause a person to use cannabis may be linked to the genes that predispose someone to schizophrenia.

Evidence consistent with this idea was reported in a study of schizophrenia patients, as well. In this study, schizophrenic patients who had reported higher rates of daily and weekly use of cannabis before illness onset had a greater number of schizophrenia-related genes in their genomes than patients who reported non-frequent use of cannabis before illness onset¹⁰Aas M, Melle I, Bettella F, et al. Psychotic patients who used cannabis frequently before illness onset have higher genetic predisposition to schizophrenia than those who did not. Psychol Med. 2018;48(1):43-49. https://www.ncbi.nlm.nih.gov/pubmed/28967348. It is therefore plausible that genetic predisposition for schizophrenia actually causes an individual to consume more cannabis. It is also plausible that cannabis use might “trigger” schizophrenia in individuals that are highly genetically susceptible.

So, what can we conclude?

The scientific data certainly support a link between cannabis use and schizophrenia. Ultimately, however, more research is necessary to understand the complex gene-environment interactions involved in the relationship between cannabis and schizophrenia, and particularly to determine causation.