Cannabis, is a widely popular recreational drug.

Still, despite its general safety, there is recent evidence that suggests that cannabis may pose a risk to adolescents whose brains have not yet matured.

And for certain teens and adults at risk for psychiatric disorders, cannabis use also has the potential to trigger a first episode of psychosis or depression.

New research suggests cannabis use can alter development of the cerebral cortex, the brain’s centre of reasoning and executive function.

A 2021 study, published in Jama Psychiatry, examined the effect of cannabis on brain development in a group of 799 adolescents, aged 13 to 17, who had a brain MRI scan at baseline and after five years of follow-up.

This is the largest longitudinal brain-imaging study to address the question. All the subjects were cannabis-naive to start, so the study could assess the impact of cannabis exposure over time.

The study found that cannabis use was associated with thinning of the prefrontal cortex and that the effect was dose-dependent – the greater the cannabis exposure, the greater the cortical thinning.

The researchers reported that accelerated cortical thinning was associated with attentional impulsivity, but there was no link with any other cognitive function. (There have been several other studies of the effect of cannabis on brain structure in teens, which reported similar findings.)

What does this mean? 

The cerebral cortex is the centre of reasoning and critical thinking; it undergoes significant structural change during adolescence and doesn't fully mature until around age 25.

We know that cannabinoid receptors are widely distributed throughout the brain and play an important role in mammalian brain development, so it is not surprising that cannabis exposure might affect brain development, especially during adolescence when the brain is uniquely sensitive to environmental influence.

We don’t yet know if these effects are enduring and whether there are any long-term psychological or cognitive effects of cannabis on young people. It’s not clear from this study exactly how much cannabis use is problematic. It seems reasonable that the risk of occasional marijuana smoking or edible use is less worrisome than daily or frequent use.

And it's important to put this hypothetical risk of cannabis in perspective. We have come a long way since the misguided moralism of “Reefer Madness”, an over-the-top propaganda film made in 1938 to discourage marijuana use.

Alcohol, when used at a certain level, is highly toxic to many organs, including the brain. And opiates are responsible for a staggering epidemic of addiction and mortality, killing millions of Americans. In contrast, there is not a single case of death from cannabis overdose.

Still, the studies should give us pause, particularly in young people, given the explosive popularity of cannabis.

Developmentally, teens are notorious risk takers and have heightened sensitivity to all kinds of rewards, like drugs, money and sex, which makes it harder for them to check their impulses.

The new data suggests that cannabis could make it even more difficult for teens to exert self-control, leading to a self-sustaining cycle of cannabis use.

Although recreational cannabis is generally safe for most people, for a small fraction of teens and adults, use of the drug can raise the risk of psychosis.

The notion that cannabis use might be associated with psychosis has been controversial and, typically, has been based on clinical anecdotes or small cross-sectional studies, which demonstrate a correlation but can’t prove a causal link.

If cannabis can cause psychosis, then increasing rates of cannabis use should be paired with a rise in the rates of psychotic illness over time.

This is just what researchers found in a nationally representative study of nearly 80 000 adults.

In the study, published in 2021 in the American Journal of Psychiatry, subjects were asked about their cannabis use and whether they had been diagnosed with schizophrenia or another psychotic disorder by their physician or other health professional.

The researchers reported an approximate 2.5-fold increase in the prevalence of self-reported psychotic illness between 2001 and 2013, which is surprising since the rates of psychotic disorder in the general population have been steady for decades.

They also found that psychotic illness was significantly more prevalent in those with any cannabis use than those without.

To be clear, the risk to any one individual was low. At the end of the study, 1.89% of those who had used cannabis reported psychotic illness compared to 0.68% in the never-used group.

The large observational study doesn’t prove a causal link between cannabis and psychosis, but it is consistent with other studies showing a similar trend.

The growing potency of cannabis

While it is always possible that the researchers did not account for other factors, aside from alcohol and stimulants, that might explain the rising rates of psychotic illness, i’'s worth noting that the trend has occurred alongside growing recreational use – and potency – of cannabis.

For example, the concentrations of THC, the active cannabinoid in marijuana, which ranged from 1.5% to 4% in the 1980s and 1990s, have soared in recent years, with some strains having as much as 29% THC. And concentrated cannabis products like “dabs” can contain upward of 76% THC.

If we accept that there is a small but real risk that cannabis can cause psychosis in some individuals, the critical question is how to identify the people.

As of yet, there is no test or biomarker for individual vulnerability for psychosis. But we do know that family psychiatric history is an important risk factor.

For example, those with a first-degree relative with a psychotic disorder like schizophrenia have an elevated genetic risk of this illness and should avoid using cannabis.

In addition to family history, low-level symptoms of a psychotic illness, such as odd behaviour, perceptual disturbances and blunted affect, could indicate a higher risk of psychosis and a need to avoid cannabis exposure.

If a teen has such risk factors, parents should have a frank talk with them about the fact that they are at a higher risk of a bad psychological and emotional reaction to cannabis.

The conversation is especially important if a teen has had an episode of psychosis or depression in response to other recreational drugs in the past. That is a tip-off that such a teen has significant psychiatric genetic risk and should avoid all recreational drugs.

I and my colleagues have seen many instances where young people with siblings or parents with a psychiatric disorder develop the first episode of psychosis or depression after cannabis exposure.

Would the disorder, for which they are at risk, never have been unleashed in the first place or perhaps developed later in life if they had not used cannabis? It's a counter-factual question that we can’t answer. But why take the chance?

Richard A Friedman is professor of clinical psychiatry and director of the Psychopharmacology Clinic at Weill Medical College of Cornell University.