The overall conversion rate over 20 years was 32.2% (95% CI, 29.7 to 34.9), with 26.0% developing schizophrenia vs 8.4% developing BD.6 Of the substances involved, cannabis was the most common, implicated in 41.2% (95% CI, 36.6 to 46.2) of cases.6 One-half of male patients converted within 2.0 years and one-half of female patients converted within 4.4 years after a cannabis-induced psychosis.6
This study had several limitations. It could not account for any short-term psychotic symptoms experienced by the general population, especially after cannabis use. Such patients might not seek treatment. Thus, the results might not be generalizable to the general population. The study did not evaluate if conversion rates differed based on continued substance use following the psychosis episode, or the amount of each substance taken prior to the episode. Dose-dependence was not well elucidated, and this study only looked at patients from Denmark and did not account for socioeconomic status.6
Another Danish study looked at the influences of gender and cannabis use in the early course of the disease in 133 patients with schizophrenia.9 These researchers found that male gender was a significant predictor of earlier onset of dysfunction socially and in the workplace, as well as a higher risk of developing negative symptoms. However, compared to gender, cannabis use was a stronger predictor of age at first psychotic episode. For cannabis users, the median age of onset of negative symptoms was 23.7, compared to 38.4 for nonusers (P < .001).9
Cannabis use is significantly elevated among individuals with psychosis, with a 12-month prevalence of 29.2% compared to 4.0% among the general population of the United States.10 In a study that assessed 229 patients with a schizophrenia spectrum disorder during their first hospitalization and 6 months, 2 years, 4 years, and 10 years later, Foti et al10 found that the lifetime rate of cannabis use was 66.2%. Survival analysis found cannabis use doubled the risk of the onset of psychosis compared to nonusers of the same age (hazard ratio [HR] = 1.97; 95% CI, 1.48 to 2.62, P < .001), even after adjusting for socioeconomic status, age, and gender (HR = 1.34; 95% CI, 1.01 to 1.77, P < .05).10 Additionally, Foti et al10 found significant positive correlations between psychotic symptoms and cannabis use in patients with schizophrenia over the course of 10 years. An increase in symptoms was associated with a higher likelihood of cannabis use, and a decrease in symptoms was correlated with a lower likelihood of use (adjusted odds ratio = 1.64; 95% CI, 1.12 to 2.43, P < .0125).10
Ortiz-Medina et al11 conducted a meta-analysis of 22 studies of 15 cohorts from healthy populations and 12 other cohort follow-up studies that evaluated the onset of psychotic symptoms in individuals who used cannabis. Most studies found associations between cannabis use and the onset of symptoms of schizophrenia, and most determined cannabis was also a major risk factor for other psychotic disorders. Analyses of dose-dependence indicated that repeated cannabis use increased the risk of developing psychotic symptoms. This risk is increased when an individual starts using cannabis before age 15.11 Age seemed to be a stronger predictor of onset and outcome than sex, with no significant differences between men and women. One study in this review found that approximately 8% to 13% cases of schizophrenia may have been solely due to cannabis.11 The most significant limitation to the studies analyzed in this review is that retrospective studies utilize self-reported questionnaires.
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