A new study suggests that the association between adolescents' cannabis consumption and psychotic disorders may be even stronger than previously thought. Earlier studies have shown that their still-developing brains are particularly vulnerable to damage by the drug.
But new research from Canada, published in the Cambridge-based journal Psychological Medicine, has now found that most adolescents diagnosed with psychotic disorder have a history of cannabis use.
A psychotic disorder is typically characterized by impaired perception of reality, explains paediatric psychiatrist Dr Rainer Thomasius, medical director of the German Centre for Addiction Research in Childhood and Adolescence, based at the Hamburg-Eppendorf University Clinic (UKE).
Sufferers have body experience disturbances and possibly visual and/or acoustic hallucinations as well, he says. Their ability to concentrate and learn are impaired, and they have a dulled capability of feeling joy or sorrow. What's more, they often feel overwhelmed by external stimuli.
Drug-related psychotic disorder can be completely cured if the person remains abstinent for a few weeks, according to Thomasius, but he warns they're at higher risk of a relapse should they resume use of the drug.
The effects of schizophrenia, often described as a type of psychotic disorder, are generally lengthier and more severe, he says. Feeling threatened, for example by family members, can lead to fatal attacks in extreme cases, he says.
Prior research on the association between youth cannabis use and psychotic disorders is based heavily on 20th-century data, when the drug was much less potent than today, the Canadian researchers note, which may explain why they found the association to be considerably greater.
In Canada, for example, the average potency of THC (tetrahydrocannabinol, one of more than 60 cannabinoids in cannabis and its principal psychoactive constituent) in illicit herbal cannabis increased from less than 1% prior to 1980 to 20% in 2018, they write.
"New types of cannabis products have also become more popular, including cannabis extracts, which can reach upwards of 95% THC."
Cannabis use in adolescence, the researchers write, is suspected to impact the endocannabinoid system - a vast network of cannabinoid receptors and chemical signals - in a way that disrupts refinement of synapses (nerve cell junctions), development of white matter (nerve fibres in deeper brain tissues), and binding to an abundant cannabinoid receptor type that regulates diverse neurotransmitters.
Along with a higher risk of psychotic disorder, Thomasius says regular cannabis use during puberty can lower IQ by about 10 points. "If an already fairly low IQ of 90 drops to 80, the result is a learning disability."
Comprehension and concentration skills can suffer too. As many as a third of all the functioning connections in the brain's frontal lobe - responsible for things such as thinking, reason and regulation of emotion - can be lost - not to mention an increased risk of anxiety disorders and depression, he adds.
What's more, regular users can also endanger lives other than their own. "Since its legalization [for recreational use] in many US states, the number of serious traffic accidents under the influence of cannabis has risen two- to 10-fold," says Thomasius, who's critical of Germany's recent legalization of the drug.
The Canadian researchers linked population-based survey data from 2009 to 2012 with records of health services covered under universal healthcare in Ontario up to 2018. The more than 11,000 respondents were aged 12-24 years at the start of the study with no prior psychotic disorder.
Roughly five in six of the adolescents (12-19 years) who were hospitalized or visited an A&E for a psychotic disorder had previously reported cannabis use. As the study relied on cannabis use at a time when recreational use was illegal for all ages in Canada, underreporting may have occurred, the researchers write.
“The vast majority of teens who use cannabis will not develop a psychotic disorder, but according to these data, most teens who are diagnosed with a psychotic disorder likely have a history of cannabis use,” lead author Dr. André McDonald is quoted as saying by the Toronto-based Centre for Addiction and Mental Health (CAMH), which collaborated on the study with the University of Toronto.
"Surprisingly, we didn't find evidence of association in young adulthood (ages 20–33 years). These findings are consistent with the neurodevelopmental theory that teens are especially vulnerable to the effects of cannabis.”
The researchers found that cannabis use, compared to no cannabis use, was associated with over 11 times greater risk of psychotic disorder at any point during adolescence.
They caution, however, that they couldn't completely rule out reverse causation, since teens with psychotic symptoms may have been self-medicating with cannabis before receiving a clinical diagnosis. Nor could they account for potentially important factors such as genetics and history of trauma.
According to Thomasius, genetics play a major role in susceptibility to psychotic disorders. If a genetic predisposition exists, he says, cannabis use can tip the scales.