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There appears to be a window of vulnerability during adolescence in which cannabis use increases the risk of developing psychotic illness. In fact, cannabis use during adolescence, but not adulthood, doubles the risk of developing schizophrenia1. While the effect of differing cannabis potency has not been studied, data shows that heavier cannabis use is associated with higher risk for eventual development of psychosis, suggesting that there is a dose-response effect2. Studies have also indicated that there is a hereditary component to this vulnerability. A genetic variant in the catechol-o-methyltransferase (COMT) gene is associated with an increased risk for schizophrenia in adolescent cannabis users3. Although there have been reports of increased prevalence of psychosis in individuals of African or Caribbean descent, there is no evidence, to our knowledge, of increased susceptibility to cannabis-induce psychosis in these ethnic populations4,5.
References:
1. Arseneault L, Cannon M, Witton J, Murray RM. Causal association between cannabis and psychosis: examination of the evidence. Br J Psychiatry. 2004;184:110-7.
2. Moore TH, Zammit S, Lingford-Hughes A, Barnes TR, Jones PB, Burke M, et al. Cannabis use and risk of psychotic or affective mental health outcomes: a systematic review. Lancet. 2007;370(9584):319-28.
3. Caspi A, Moffitt TE, Cannon M, McClay J, Murray R, Harrington H, et al. Moderation of the effect of adolescent-onset cannabis use on adult psychosis by a functional polymorphism in the catechol-O-methyltransferase gene: longitudinal evidence of a gene X environment interaction. Biol Psychiatry. 2005;57(10):1117-27.
4. Kantrowicz J, Nolan K, et al. Adolescent Cannabis Use, Psychosis and Catechol-O-Methyltransferase Genotype in African Americans and Caucasians. Psychiatric Quarterly. 2009; 80(4): 213-218
5. Cantor-Graee E. Ethnic minority groups, particularly African-Caribbean and Black African groups, are at increased risk of psychosis in the UK. Evid Based Ment Health. 2007 Aug;10(3):95
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