Other researchers have found it would take a relatively high number of individuals to stop using cannabis to prevent 1 case of schizophrenia. In a study of data from England and Wales, Hickman et al12 evaluated the best available estimates of the incidence of schizophrenia, rates of heavy and light cannabis use, and risk that cannabis causes schizophrenia to determine the number needed to prevent (NNP) 1 case of schizophrenia. They estimated that it would require approximately 2,800 men age 20 to 24 (90% CI, 2,018 to 4,530) and 4,700 men age 35 to 39 (90% CI, 3,114 to 8,416) who heavily used cannabis to stop their consumption to prevent 1 case of schizophrenia.12 For women with heavy cannabis use, the mean NNP was 5,470 for women age 25 to 29 (90% CI, 3,640 to 9,839) and 10,870 for women age 35 to 39 (90% CI, 6,786 to 22,732).12 For light cannabis users, the NNP was 4 to 5 times higher than the NNP for heavy cannabis users. This suggests that clinical interventions aimed at preventing dependence on cannabis would be more effective than interventions aimed at eliminating cannabis use.
Medical cannabis and increased potency
In recent years, the use of medical cannabis, which is used to address adverse effects of chemotherapy as well as neuropathic pain, Parkinson’s disease, and epilepsy, has been increasing.13 However, there is a lack of well-conducted randomized clinical trials evaluating medical cannabis’ efficacy and safety. As medical cannabis continues to gain public acceptance and more states permit its legal use, patients and physicians should be fully informed of the known adverse effects, including impaired attention, learning, and motivation.13
Several studies have drawn attention to the dose-dependence of many of cannabis’ effects. Since at least the 1960s, the concentration of THC in cannabis has increased substantially, thus increasing its potency. Based on 66,747 samples across 8 studies, 1 meta-analysis estimated that THC concentrations in herbal cannabis increased by 0.29% (P < .001) each year between 1970 and 2017.14 Similarly, THC concentrations in cannabis resins were found to have increased by 0.57% (P = .017) each year between 1975 and 2017.14 Cannabis products with high concentrations of THC carry an increased risk of addiction and mental health disorders.14
Identifying those at highest risk
Despite ongoing research, scientific consensus on the relationship of cannabis to schizophrenia and psychosis has yet to be reached. The disparity between the relatively high prevalence of regular adult use of cannabis (8%7) and the low incidence of cannabis-induced psychosis suggests that cannabis use alone is unlikely to lead to episodes of psychosis in individuals who are not predisposed to such episodes. Sarrazin et al15 evaluated 170 patients with schizophrenia, 31 of whom had cannabis use disorder. They found no significant difference in lifetime symptom dimensions between groups, and proposed that cannabis-associated schizophrenia should not be categorized as a distinct clinical entity of schizophrenia with specific features.15
Policies that encourage follow-up of patients after episodes of drug-induced psychosis may mitigate the adverse social and economic effects of schizophrenia. Currently, these policies are not widely implemented in health care institutions, possibly because psychotic symptoms may fade after the drug’s effects have dissipated. Despite this, these patients are at high risk of developing schizophrenia and self-harm. New-onset schizophrenia should be promptly identified because delayed diagnosis is associated with worse prognosis.6 Additionally, identifying genetic susceptibilities to schizophrenia—such as the Val158Met polymorphisms—in individuals who use cannabis could help clinicians manage or slow the onset or progression of schizophrenia.3 Motivational interviewing strategies should be used to minimize or eliminate cannabis use in individuals with active schizophrenia or psychosis, thus preventing worse outcomes.
Bottom Line
Identifying susceptibilities to schizophrenia may guide interventions in patients who use cannabis. Several large studies have suggested that cannabis use may exacerbate symptoms and worsen the prognosis of schizophrenia. Motivational interviewing strategies aimed at minimizing cannabis use may improve outcomes in patients with schizophrenia.
Related Resources
- Khokhar JY, Dwiel LL, Henricks AM, et al. The link between schizophrenia and substance use disorder: a unifying hypothesis. Schizophr Res. 2018;194:78-85. doi:10.1016/j. schres.2017.04.016
- Otite ES, Solanky A, Doumas S. Adolescents, THC, and the risk of psychosis. Current Psychiatry. 2021;20(12):e1-e2. doi:10.12788/cp.0197