Daily Use, Especially of High-Potency Cannabis, Drives the Earlier Onset of Psychosis in Cannabis Users
Marta Di Forti, Hannah Sallis, Fabio Allegri, Antonella Trotta, Laura Ferraro, Simona A. Stilo, Arianna Marconi, Caterina La Cascia, Tiago Reis Marques, Carmine Pariante, Paola Dazzan, Valeria Mondelli, Alessandra Paparelli, Anna Kolliakou, Diana Prata, Fiona Gaughran, Anthony S. David, Craig Morgan, Daniel Stahl, Mizanur Khondoker, James H. MacCabe and Robin M. Murray
December 17, 2013
Cannabis use is associated with an earlier age of onset of psychosis (AOP). However, the reasons for this remain debated. Methods: We applied a Cox proportional hazards model to 410 first-episode psychosis patients to investigate the association between gender, patterns of cannabis use, and AOP. Results: Patients with a history of cannabis use presented with their first episode of psychosis at a younger age (mean years = 28.2, SD = 8.0; median years = 27.1) than those who never used cannabis (mean years = 31.4, SD = 9.9; median years = 30.0; hazard ratio [HR] = 1.42; 95% CI: 1.16–1.74; P < .001). This association remained significant after controlling for gender (HR = 1.39; 95% CI: 1.11–1.68; P < .001). Those who had started cannabis at age 15 or younger had an earlier onset of psychosis (mean years = 27.0, SD = 6.2; median years = 26.9) than those who had started after 15 years (mean years = 29.1, SD = 8.5; median years = 27.8; HR = 1.40; 95% CI: 1.06–1.84; P = .050). Importantly, subjects who had been using high-potency cannabis (skunk-type) every day had the earliest onset (mean years = 25.2, SD = 6.3; median years = 24.6) compared to never users among all the groups tested (HR = 1.99; 95% CI: 1.50- 2.65; P < .0001); these daily users of high-potency cannabis had an onset an average of 6 years earlier than that of non-cannabis users. Conclusions: Daily use, especially of high-potency cannabis, drives the earlier onset of psychosis in cannabis users.