A new study claims to have identified a genetic link that connects cannabis use with schizophrenia, but some experts are casting doubt on the study and its methods.
The study, published in the September 2018 issue of Nature Neuroscience, also identified 35 genes significantly linked with cannabis use. The researchers also found genetic correlations of cannabis use with 25 traits, including those related to substance use, personality and mental health. For example, risk-taking behavior, openness to experience and educational attainment correlated with cannabis use.
“The study was designed to identify common genetic variants that influence the likelihood that someone has ever used cannabis,” one of the study authors, Abraham Palmer, Ph.D., professor and vice chair for basic research in the Department of Psychiatry at the University of California, San Diego, told Marijuana.com. “We identified eight such loci, including one that contains the gene CADM2, which has been identified by several recent studies examining genetic modulators of risk tolerance.”
The study is the largest of its kind to date with a sample size of 184,765, five times larger than the previous largest genome-wide association study (GWAS) on lifetime cannabis use, which the researchers defined as any use of cannabis during a person's lifetime. The researchers were able to acquire the substantial sample size by using data from the International Cannabis Consortium, UK Biobank, and 23andMe, a personal genomics company that provides health, trait and ancestry reports based on an individual's DNA.
“As a scientist, I try to be cautious about jumping to conclusions about the causal direction of a correlation, in this case, the correlation between schizophrenia and cannabis use,” Palmer said. “So I was not that surprised when the data did not support the idea that cannabis use increased the risk of developing schizophrenia. But I suspect that the conclusion is surprising to others.”
However, Mitch Earleywine, a psychology professor at the State University of New York at Albany, author of “Understanding Marijuana” and National Organization for the Reform of Marijuana Laws (NORML) advisory board member, isn't surprised – especially with the findings related to cannabis use and education.
“As the authors state directly, lifetime cannabis use correlates positively and significantly with lifetime educational attainment, higher income, and more intelligence,” Earleywine told Marijuana.com.
The study wasn't without its limitations. While the sample size was substantial, a larger sample size would be even more useful.
“So far we have only studied whether people had ever tried cannabis, which lumps together people who tried cannabis once in college with people who used cannabis every day for 25 years,” Palmer explained. “Trying cannabis tells us nothing about whether someone likes the effects of the drug, but, presumably, heavy use occurs only in people who like the effects.”
Drug Policy Experts Express Skepticism
Sheila P. Vakharia, policy manager for the decriminalization group Drug Policy Alliance, told Marijuana.com that the study is flawed because it establishes as a major variable any reported amount of marijuana use, an overly broad category. The study defined cannabis use to include people who have used cannabis once, people who experimented, people who use socially, and people who have cannabis use disorder.
“This study lumps all those different groups together in their analysis because all of those people admitted to using at least once in their lives,” Vakharia said. “I think it's really important not to conflate use with misuse, abuse, dependence, addiction. But substance use is not indicative of harm or that someone has a problem. In fact, most people who use substances never develop actual substance use disorders or problems.”
Vakharia also took issue with the study's correlation between genetics and substance use.
“Cannabis use doesn't just happen in a bubble out of thin air,” Vakharia said. “To say that drug use was predicted by genetics minimizes the fact that people are autonomous beings who make decisions based on how they're feeling in the moment, their own pre-existing mental health experiences, their physical health experiences, their expectations, their self esteem.”
But perhaps the most significant factor is, according to the study's abstract, that, “All measured genetic variants combined explained 11 percent of the variance.” To Vakharia, that means the study's model doesn't stand up to scrutiny.
“That means that 89 percent of the reasons why people use marijuana are explained by other factors,” Vakharia said with a laugh. “Other life factors: circumstances, choice, autonomy … life!”
From a policy perspective, Mason Tvert, media relations director for the anti-prohibition group Marijuana Policy Project, said it's important to focus on those with existing mental health issues rather than the general population.
“Rather than having warnings that marijuana could cause schizophrenia that the evidence does not back up, we should have warnings that people who already have schizophrenia … should be aware of the potential risks of using cannabis,” Tvert told Marijuana.com.
And while Vakharia said she was interested in the finding that suggests schizophrenia risk may precede marijuana use, she ultimately didn't find the study very useful.
“What I think is clinically useful, practically useful, and important to most people is, 'How do we protect people against escalating from occasional, recreational, social use to problematic use?' That is clinically useful information,” Vakharia said. We know that people are going to get more access to marijuana. What are the best strategies to help people to stay within their limits and lead more balanced lives?”