The use of marijuana is associated with an earlier onset of psychosis, and that association might be causal, a meta-analysis published Feb. 7 online in Archives of General Psychiatry has shown.
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A recent metanalysis of 83 studies linked marijuana use with earlier onset of psychosis.
"This study lends weight to the view that cannabis use precipitates schizophrenia and other psychotic disorders, perhaps by an interaction between genetic and environmental factors ... or by disrupting brain development, especially during the important neurological maturation that takes place during adolescence," said Dr. Matthew Large of Prince of Wales Hospital and the University of New South Wales, Sydney, and his associates.
Further, the study findings raise "important questions of whether cannabis ... can trigger psychosis by direct neurotoxic effects, by alterations in dopamine activity, or by other changes in neurotransmission and the extent to which any adverse effects on the brain are reversible." Future research should focus on finding "the mechanisms by which cannabis use triggers or brings forward psychotic illness," the investigators added.
They performed a meta-analysis of 83 studies that reported age at onset of psychotic disorders in cohorts of patients in which the use of psychoactive substances also was recorded. These included 8,167 patients who reported that they used substances and 14,352 who reported that they did not.
Overall, the mean age of psychosis onset in patients who used substances was about 2 years younger than the age of psychosis onset in patients who did not.
In a further analysis, study samples of patients who used cannabis in particular showed an onset of psychosis that was nearly 3 years earlier than in study samples of patients who did not use cannabis. Similarly, the onset of psychosis was 2 years earlier in samples of patients who used unspecified psychoactive substances than in samples of those who did not.
In contrast, the use of alcohol alone was not significantly associated with a younger age of onset of psychosis. This finding refutes the possibility that "people with a propensity to develop psychosis at a younger age are simply more likely to use all substances," Dr. Large and his colleagues said (Arch. Gen. Psychiatry 2011 Feb. 7 [doi:10.1001/archgenpsychiatry.2011.5]).
Several other variables also showed no association with age at onset of psychosis, including the severity of substance use, how a study measured the onset of psychosis, which instruments were used to diagnose psychosis or substance use, whether the study included only patients with first-episode psychosis, and the year in which the study was conducted.
The results suggest that reducing marijuana use could delay or even prevent some cases of psychosis. Even if reducing cannabis use only delayed the onset of a psychotic illness that was inevitable, "an extra 2 or 3 years of psychosis-free functioning could allow many patients to achieve the important developmental milestones of late adolescence and early adulthood that could lower the long-term disability arising from psychotic disorders," the researchers noted.
These findings warrant "a renewed public health warning about the potential for cannabis use to bring on psychotic illness," they added.
The authors reported no relevant financial disclosures.