A recent twin study found limited evidence of a direct causal relationship between moderate cannabis use and most negative psychological outcomes, suggesting that shared genetic and environmental factors largely explain these associations. However, it identified modest yet significant effects of cannabis use on specific areas, including illicit drug use. The findings have been published in the Journal of Psychopathology and Clinical Science.
“Cannabis is recreationally legal for adult use in many states now,” said study author Stephanie Zellers, a postdoctoral researcher at the Institute for Molecular Medicine Finland at the University of Helsinki. “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.”
The study analyzed data from approximately 4,078 individuals, drawn from two longitudinal community twin samples maintained by the Minnesota Center for Twin Family Research and the Colorado Center for Antisocial Drug Dependence. These participants were initially recruited during adolescence through birth records and had been born between 1972 and 1994. Their cannabis use was prospectively tracked from ages 11 to 35, spanning the years 1994 to 2014.
The primary measure of interest was the “lifetime average cannabis frequency,” which represented an average frequency of cannabis use across all available waves of assessment. This measure aimed to capture the cumulative effects of cannabis use throughout adolescence and young adulthood. Additionally, the study explored alternative measures of cannabis exposure to consider different conceptualizations of cannabis use, including age of onset and patterns of heavy use.
Outcomes assessed in adulthood included a wide range of substance use, psychiatric symptoms, and psychosocial factors. These outcomes were measured through self-report surveys and structured clinical interviews, providing insights into the participants’ substance use habits (including alcohol and tobacco use), psychiatric conditions (like cannabis use disorder symptoms and disordered personality traits), and various aspects of psychosocial functioning (e.g., employment status, financial problems, cognitive ability).
Central to their methodology was the cotwin control study design, a quasi-experimental approach that compares monozygotic twins who share nearly 100% of their genetic makeup. This design capitalizes on the natural experiment afforded by twins who differ in their exposure to cannabis, effectively controlling for genetic and many shared environmental factors that could confound the observed relationships between cannabis use and various outcomes.
While cannabis use correlated with several adverse outcomes at the individual level, the researchers found that these associations largely dissipated when examining differences within twin pairs. This pattern suggested that many of the negative outcomes traditionally associated with cannabis use might not be directly caused by the substance itself but are instead attributable to shared genetic liabilities and environmental influences.
In other words, individuals might be predisposed to both cannabis use and certain adverse outcomes due to these underlying factors, challenging the notion that cannabis use is a primary cause of these issues.
But the study’s findings were not uniformly dismissive of cannabis’s potential adverse effects. A notable exception emerged in the relationship between cannabis use and specific outcomes such as cannabis use disorder, tobacco use frequency, and illicit drug involvement.
In these instances, the researchers observed modest but statistically significant effects that persisted even within twin pairs. For example, twins who used cannabis more frequently tended to have higher rates of cannabis use disorder symptoms compared to their lesser-using siblings, indicating that increased cannabis use might indeed contribute to the likelihood of developing problematic use patterns.
“Higher lifetime average cannabis use causes increased symptoms of cannabis use disorder,” Zellers 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.”
“Individuals who choose to use cannabis should do so in moderation, learn about the risk of cannabis use disorder, and be mindful of their use of other substances that could cause physical or psychological harm.”
However, there are some caveats to consider. “The average frequency of cannabis use in our sample was about once or twice a month, and so our results cannot tell us as much about the potential consequences of heavy cannabis use,” Zellers noted.
“It is possible that relatively infrequent use does not cause psychological harm, but heavier use has more direct consequences. We also have a community sample with low rates of psychopathology, it is possible that there are some individuals who are more likely to experience negative consequences of cannabis use.”
“For example, some people are concerned that cannabis use could cause psychosis or worsening symptoms in people predisposed to schizophrenia. We could not examine this withy our study. Lastly, we only looked at a subset of psychological outcomes, there are many things, like physical health, that could be affected by cannabis use that we did not study here.”
The study, “Limited Psychological and Social Effects of Lifetime Cannabis Use Frequency: Evidence From a 30-Year Community Study of 4,078 Twins,” was authored by Stephanie Zellers, Jordan Alexander, Jarrod M. Ellingson, Jonathan D. Schaefer, Robin P. Corley, William Iacono, John K. Hewitt, Christian J. Hopfer, Matt K. McGue, and Scott Vrieze.
