A study involving adult twins has found little evidence that ‘moderate’ cannabis causes negative effects on mental health.
The study, published in the Journal of Psychopathology and Clinical Science, analysed a sample of 4,078 American adult twins over 30 years to investigate the effects of lifetime cannabis use. Researchers assessed twins who lived together as children and shared either 50% or 100% of their genes, comparing twins who had different levels of exposure to cannabis. By using this method, the study was able to account for genetic and shared environmental factors that could lead to outcomes sometimes correlated to cannabis use.
Participants were assessed for various outcomes, such as substance use habits and psychiatric conditions as well as psychosocial functioning such as employment status, financial issues, and cognitive ability.
While the study did find some relationship between cannabis use and adverse outcomes in individuals, the researchers found that when compared to the co-twin the outcomes were likely due to genetics or the environment shared while growing up. “This study suggests that lifetime exposure to cannabis has few persistent effects on mental health and other psychosocial outcomes,” researchers said.
The study also found a relationship between cannabis use and cannabis use disorder, tobacco use frequency, and other illicit drug use. Twins who used cannabis more frequently tended to have higher rates of cannabis use disorder symptoms compared to their lesser-using siblings.
“Higher lifetime average cannabis use causes increased symptoms of cannabis use disorder,” study author Stephanie Zellers, a postdoctoral researcher at the Institute for Molecular Medicine Finland at the University of Helsinki told PsyPost. “There is also some evidence that increased cannabis use may also cause increased use of tobacco and illicit drugs, but the causal relationship was less clear. Besides those things, we did not find that lifetime average cannabis use causes significant harm in the psychological and psychiatric outcomes that we investigated.”
“Cannabis is recreationally legal for adult use in many states now,” Zellers said. “I have studied the consequences of recreational cannabis legalization and found that cannabis legalization causes increased cannabis use. This is potentially a reason for concern, because cannabis use is associated with a variety of negative outcomes.”
“That said, correlation does not imply causation! So I wanted to investigate the causal links between cannabis use and psychological and psychiatric outcomes. I believe adults should have access to rigorous and causally informative science to make informed choices about their own use of legal substances and the possible consequences.”
