title: Daily use, especially of high-potency cannabis, drives the earlier onset of psychosis in cannabis users.
abstract: 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.
authors.affiliation: Department of Psychosis Studies, Institute of Psychiatry, Kings College London, London, UK; marta.diforti@kcl.ac.uk.
authors.forename: Marta
authors.initials: M
authors.lastname: Di Forti
authors.affiliation: School of Social and Community Medicine, Bristol University, Bristol, UK;
authors.forename: Hannah
authors.initials: H
authors.lastname: Sallis
authors.affiliation: Department of Psychiatry, Bologna University, Bologna, Italy;
authors.forename: Fabio
authors.initials: F
authors.lastname: Allegri
authors.affiliation: Department of Psychosis Studies, Institute of Psychiatry, Kings College London, London, UK;
authors.forename: Antonella
authors.initials: A
authors.lastname: Trotta
authors.affiliation: Department of Psychiatry and Neuroscience, Palermo University, Palermo, Italy;
authors.forename: Laura
authors.initials: L
authors.lastname: Ferraro
authors.affiliation: Department of Health Services and Public Health, Institute of Psychiatry, Kings College London, London, UK;
authors.forename: Simona A
authors.initials: SA
authors.lastname: Stilo
authors.affiliation: Department of Psychosis Studies, Institute of Psychiatry, Kings College London, London, UK;
authors.forename: Arianna
authors.initials: A
authors.lastname: Marconi
authors.affiliation: Department of Psychiatry and Neuroscience, Palermo University, Palermo, Italy;
authors.forename: Caterina
authors.initials: C
authors.lastname: La Cascia
authors.affiliation: Department of Psychosis Studies, Institute of Psychiatry, Kings College London, London, UK;
authors.forename: Tiago
authors.initials: T
authors.lastname: Reis Marques
authors.affiliation: Department of Psychological Medicine, Institute of Psychiatry, Kings College London, London, UK;
authors.forename: Carmine
authors.initials: C
authors.lastname: Pariante
authors.affiliation: Department of Psychosis Studies, Institute of Psychiatry, Kings College London, London, UK;
authors.forename: Paola
authors.initials: P
authors.lastname: Dazzan
authors.affiliation: Department of Psychological Medicine, Institute of Psychiatry, Kings College London, London, UK;
authors.forename: Valeria
authors.initials: V
authors.lastname: Mondelli
authors.affiliation: Department of Psychosis Studies, Institute of Psychiatry, Kings College London, London, UK;
authors.forename: Alessandra
authors.initials: A
authors.lastname: Paparelli
authors.affiliation: Department of Psychosis Studies, Institute of Psychiatry, Kings College London, London, UK;
authors.forename: Anna
authors.initials: A
authors.lastname: Kolliakou
authors.affiliation: Department of Psychosis Studies, Institute of Psychiatry, Kings College London, London, UK;
authors.forename: Diana
authors.initials: D
authors.lastname: Prata
authors.affiliation: Department of Psychosis Studies, Institute of Psychiatry, Kings College London, London, UK;
authors.forename: Fiona
authors.initials: F
authors.lastname: Gaughran
authors.affiliation: Department of Psychosis Studies, Institute of Psychiatry, Kings College London, London, UK;
authors.forename: Anthony S
authors.initials: AS
authors.lastname: David
authors.affiliation: Department of Health Services and Public Health, Institute of Psychiatry, Kings College London, London, UK;
authors.forename: Craig
authors.initials: C
authors.lastname: Morgan
authors.affiliation: Department of Biostatistics, Institute of Psychiatry, Kings College London, London, UK.
authors.forename: Daniel
authors.initials: D
authors.lastname: Stahl
authors.affiliation: Department of Biostatistics, Institute of Psychiatry, Kings College London, London, UK.
authors.forename: Mizanur
authors.initials: M
authors.lastname: Khondoker
authors.affiliation: Department of Psychosis Studies, Institute of Psychiatry, Kings College London, London, UK;
authors.forename: James H
authors.initials: JH
authors.lastname: MacCabe
authors.affiliation: Department of Psychosis Studies, Institute of Psychiatry, Kings College London, London, UK;
authors.forename: Robin M
authors.initials: RM
authors.lastname: Murray
mesh_terms: D000328:Adult; D000341:Affective Disorders, Psychotic / Q000453:epidemiology*; D017668:Age of Onset; D002188:Cannabis / Q000009:adverse effects*; D005260:Female; D006801:Humans; D008297:Male; D011618:Psychotic Disorders / Q000453:epidemiology*; D012306:Risk; D012737:Sex Factors
keywords: age of onset; cannabis; drug use; gender; high-potency cannabis; psychotic disorders; survival plots