Twins who smoked weed mental health
Twins who smoked weed mental health

4,000 Pairs of Twins Were Studied, 1 Smoked Weed and 1 Didn't - Cannabis Use Did Not Cause Any Negative Mental Health Outcomes

A lifetime study of twins showed no negative mental health effects of using marijuana.

Posted by:
DanaSmith on Tuesday Feb 13, 2024

twins who smoke weed mental health study

A variety of different study models have been used over the last few years when it comes to cannabis.

However, twin studies in particular lifetime twin studies are among the most useful and effective when it comes to understanding cannabis on a deeper level. Generally speaking, twin studies have numerous advantages in research – most especially because these greatly increase the statistical impact of any genetic study by decreasing the environmental and genetic variability.

Since twins possess the same genes though oftentimes their environments can differ as they age, twin studies help shed light on the impact of environmental and genetic contributions toward a factor.


In the case of one recent paper, a lifetime twin study shows that marijuana use doesn’t have negative mental health outcomes.

Let’s take a deeper look into its significance.


Researchers from the University of Colorado and the University of Minnesota analyzed the long-term effects of marijuana on psychosocial and psychiatric outcomes on a population of over 4,000 adult twins. They were studied from 1994 through 2021.


“This study suggests that lifetime exposure to cannabis has few persistent effects on mental health and other psychosocial outcomes,” said the researchers. “We did not identify within-pair differences in cognitive ability… Cannabis consumption did not predict within-pair differences in psychoticism,” they added. In effect, lifetime consumption of weed didn’t lead to negative mental health outcomes among the sample.

“Broadly speaking, our results do not support a causal relationship between lifetime average cannabis frequency and most of the substance use, psychiatric, and psychosocial outcomes assessed here. Rather, genetic and familial confounding most likely explain the relationships between cannabis use and the negative outcomes associated with it,” concluded the authors. “The lack of within-pair effects, or small effects for those existing within-pair differences, in our primary outcome suggests that cumulative cannabis use does not have large, or lasting effects on many psychosocial outcomes,” the authors concluded.


Other Twin Studies On Cannabis


There have been other similar twin studies conducted, sharing similar results.


A 2016 twin study, conducted by researchers from the University of California and the University of Minnesota sought to understand if cannabis was linked to changes in IQ levels. The participants were then tested using various intelligence benchmarks from the ages of 9 to 12, which was before they began consuming weed, then again from 17 to 20 years old.


According to the researchers, there were no dose-specific relationships associated with IQ decline and cannabis use. “In the largest longitudinal examination of marijuana use and IQ change, we find little evidence to suggest that adolescent marijuana use has a direct effect on intellectual decline… The lack of a dose-response relationship, and an absence of meaningful differences between discordant siblings lead us to conclude that the deficits observed in marijuana users are attributable to confounding factors that influence both substance initiation and IQ rather than a neurotoxic effect of marijuana,” they said.


They did find that cannabis users among the twins resulted in a reduction of 4 IQ points during the course of the study. But, it’s important to note that there was a similar decline pattern observed among the twins gradually, which suggests that there were other factors at play affecting the IQ decrease.


In conclusion, cannabis use doesn’t affect IQ levels, as prohibitionists love to say! They also frequently argue that cannabis use makes you stupid, yet they lack the studies and evidence to back up these claims.


Meanwhile, another twin study from September 2021 was conducted by investigators from the University of Minnesota Institute of Child Development. They assessed the relationship between adult-onset psychosis and cannabis use in adolescence, using a longitudinal co-twin study, concluding that exposure to cannabis wasn’t linked to a schizophrenia risk later on.


“Epidemiological studies have repeatedly shown that individuals who use cannabis are more likely to develop psychotic disorders than individuals who do not. It has been suggested that these associations represent a causal effect of cannabis use on psychosis, and that psychosis risk may be particularly elevated when use occurs in adolescence,” they concluded. “This study, however, does not support these hypotheses, suggesting instead that observed associations are more likely due to confounding by common vulnerability factors.”

“The results suggest this association is likely attributable to familial confounds rather than a causal effect of cannabis exposure,” the researchers conclude. “Our results suggest that the threat of potential harm to adolescents via meaningful increases in risk of long-term psychotic illness may be overstated,” they concluded.


There are other studies on this subject clearly concluding that cannabis use alone doesn’t cause psychosis, which is what many prohibitionists are claiming. However, we must note that if one already has schizophrenic tendencies, one must abstain from cannabis or other psychoactive drugs unless taken with the guidance of a medical professional. There are several other factors at play, and certainly if one already has a predisposition to specific mental illnesses, self-medicating with any kind of drug is not recommended.




Twin studies such as these mentioned above add credibility to other studies assessing similar topics surrounding cannabis, as well as other health issues. The use of twin studies is significant for controlling genetics, which is one of the most important variables to consider when assessing health.

These studies can help us learn valuable amounts of information and can change our understanding of how cannabis impacts our bodies, for the better!





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