In November of 2017 Canada’s House of Commons passed legislation to legalize recreational marijuana in Canada. The bill is now in the Canadian Senate, which is a “wild card,” according to University of Toronto political scientist, Nelson Wiseman. “It’s a wild card because you’ve got all these independents and you don’t know what they’re going to say, how engaged they are with the issue. Some might fight it tooth and nail, some might try delay tactics as individual members,” he said. The slow pace of debate in the Senate has led to impatience among supporters of Bill C-45. Perhaps the delay will allow some consideration of how the Canadian government will address findings by researchers at the University of Montreal that links marijuana use and psychotic-like experiences (PLE) in Canadian adolescents.

In an article published in The Journal of Child Psychology and Psychiatry in July of 2017, Bourque et al. sought to investigate whether there was a longitudinal relationship between cannabis use and PLE. The researchers found that a steeper growth in cannabis use between the ages of 13 and 16 was associated with a higher likelihood of recurrent PLEs. The study’s lead author, Josiane Bourque, was quoted by Science Daily as saying: “Our findings confirm that becoming a more regular marijuana user during adolescence is, indeed, associated with a risk of psychotic symptoms. This is a major public-health concern for Canada.” Going from an occasional user to a weekly or daily user increased an adolescent’s risk of having PLEs by 159%. The link between marijuana use and PLEs was best explained by emerging symptoms of depression.

There were also only mild effects of marijuana use on measures of cognitive development, specifically with response inhibition performance. “Our results show that apart from a marginal effect of response inhibition, there was no association between change in cognitive functioning and PLE trajectory membership.” These findings were inconsistent with the emerging literature on adolescents and PLE. The researchers speculated the difference in their findings may be because their study used longitudinal, multi-level modeling to examine the relationship between cannabis use and PLE among adolescents. However, “these results are in accordance with recent findings from our team demonstrating that cannabis use has a neurotoxic effect that is specific to response inhibition, as opposed to a general impact on cognitive functioning.”

The clinical implications of these results nevertheless highlight the need for reducing cannabis use in high risk adolescents, as well as the importance of addressing depressive symptoms in programs aimed at preventing increasing PLE in high risk youth.

The Canadian study isn’t the only recent research linking psychosis and marijuana use in adolescents. Carney et al. at the University of Manchester did a meta-analysis in “Cannabis use and symptom severity in individuals at ultra high risk for psychosis.” They sought to investigate whether ultra high risk (UHR) individuals have higher rates of current and lifetime cannabis use than healthy controls (HCs); whether UHR individuals have higher rates of cannabis use disorders (CUD) than HCs; and do UHR cannabis users have higher positive and negative symptoms than non-cannabis using UHR subjects.

UHR individuals had high rates of cannabis use. They found that 52.8% of UHR individuals reported using cannabis in their lifetime. “Our analyses found significantly higher rates of lifetime cannabis use in the UHR samples than in the HC groups.” Approximately one in four UHR individuals were currently using cannabis. They also found high rates of comorbid CUDs in UHR individuals. “Even prior to the onset of psychosis, UHR individuals are likely to engage in risky cannabis use.”

High rates of cannabis use in this group are perhaps unsurprising given that use of substances is common in young people who present for mental health care and people with early psychosis. As there is evidence to suggest frequent use of high‐potency cannabis increases the risk for later transition, it is important that early intervention services encourage substance use reduction upon first presentation. A previous review and meta‐analysis found that UHR individuals are significantly more likely to smoke, abuse alcohol and have lower levels of physical activity than their peers. Here, we add to this evidence to suggest that this group is also more likely to have used cannabis or have a CUD, posing an additional risk factor to both physical and mental health.

Carney et al. found there was a statistically significant relationship between UHR cannabis use and severe positive symptoms of psychosis such as unusual thought content and suspiciousness. This was consistent with previous research with first episode psychosis individuals, where cannabis use was associated with increased positive symptoms of psychosis (i.e., hallucinations, suspiciousness and delusions) and poorer psychosocial functioning and long-term outcome. They were not able to analyze individual negative symptoms because of a lack of available data. They speculated that positive symptoms of psychosis may occur as a direct result of substance use. “Indeed, cannabis can induce symptoms of psychosis in healthy populations, and may therefore influence symptom severity in the UHR group.”

Irrespective of causation, high rates of cannabis use in the UHR group carries important clinical implications. Although many UHR individuals will not develop full‐threshold psychosis, they may go on to have anxiety, mood or substance use disorders, and continue to function poorly regardless of transition or symptomatic remission. Therefore, it is important to address any comorbid disorders at an early stage. Future research should assess the efficacy of interventions used to reduce cannabis use in UHR individuals upon first presentation to mental health services. For example, motivational interviewing and cognitive behaviour therapy have been found to be effective in reducing cannabis use among early psychosis groups, although a randomised control trial in the UHR group is yet to be conducted. Longitudinal studies are also required to highlight any relationship between continued cannabis use and factors such as long‐term outcome, functioning and symptoms over time.

High THC levels in cannabis are associated with psychosis. Writing for Scientific American, R. Douglas Fields reported on research presented at the World Psychiatric Conference that teenage cannabis use hastens the onset of schizophrenia in vulnerable individuals. Hannelore Ehrenreich presented results of a study of 1,200 people with schizophrenia. “The results … show people who had consumed cannabis before age 18 developed schizophrenia approximately 10 years earlier than others. The higher the frequency of use, the data indicated, the earlier the age of schizophrenia onset.” Ehrenreich said: “Cannabis use during puberty is a major risk factor for schizophrenia,”

Robin Murray, a professor of psychiatry at King’s College London, was one of the first scientists to research the link between cannabis and schizophrenia. Speaking at the conference, he cited 10 studies that found a significant risk of young marijuana users developing psychosis. “The more [cannabis] you take—and the higher the potency—the greater the risk.” His research with users in London has shown that high potency cannabis (about 16% THC) was involved in 24% of all cases having a first episode of psychosis.

Another speaker at the conference, Beat Lutz, a neurochemist, described how marijuana might produce adverse effects in a young person’s brain. THC disrupts the normal flow of signals among brain cells—“a process normally regulated by chemicals called endocannabinoids.” These occur naturally in the body and activate the CB₁ or cannabinoid type 1 receptor. The CB₁ receptor acts like a circuit breaker in the brain, keeping its signaling activity or “excitation” within a normal range.

Too little endocannabinoid signaling results in excessive excitation of the nervous system, and this can promote anxiety disorders, impulsivity and epilepsy. Too much activity has the opposite effect and can promote depression, for example. Upsetting the information flows regulated by the endocannabinoid system has also been linked to psychosis.

THC acts differently than the naturally occurring endocannabinoids. It doesn’t break down rapidly in the body the way endocannabinoids do. This sustained activation causes serious wide-ranging problems in the brain. Low doses of THC might reduce anxiety, but high doses can heighten it. Chronic overstimulation of CB₁ receptors by THC shuts down the body’s natural endocannabinoid signaling system by eliminating the CB₁ receptors from neurons. Lutz also believes:

THC’s disruption of endocannabinoid signaling in the early teen brain can hinder key neurodevelopmental processes that involve the CB₁ receptors, thereby impairing brain communication permanently.

Researchers at the conference commented on the need for the public to become aware of these new findings. Peter Falkai, a psychiatrist at the Munich Center for Neurosciences at Ludwig Maximilian University, said: “Looking into the data, clearly yes, the data show increasing risk of psychosis.”