title: Effects of continuation, frequency, and type of cannabis use on relapse in the first 2 years after onset of psychosis: an observational study.
abstract: BACKGROUND
  Although cannabis use after a first episode of psychosis has been associated with relapse, little is known about the determinants of this most preventable risk factor for relapse of psychosis. Here we aimed to study whether the effects on outcome vary depending on the type of cannabis consumed and usage pattern.
  
  
  METHODS
  In this observational study, we prospectively recruited and followed up patients aged 18-65 years who presented with their first episode of psychosis to psychiatric services in south London, London, UK. Relapse of psychosis within 2 years after onset of psychosis was defined as risk of subsequent admission to hospital. We classified patients into different patterns of cannabis use based on continuity of use after onset of psychosis, potency of cannabis consumed, and frequency of use after the onset of their illness. We used multiple regression analyses (logistic or binominal) to compare the different cannabis use groups and propensity score analysis to validate the results.
  
  
  RESULTS
  Between April 12, 2002, and July 26, 2013, 256 patients presented with a first episode of psychosis. We did follow-up assessments for these patients until September, 2015. Simple analyses showed that former regular users of cannabis who stopped after the onset of psychosis had the most favourable illness course with regards to relapse. In multiple analysis, continued high-frequency users (ie, daily use in all 24 months) of high-potency (skunk-like) cannabis had the worst outcome, indexed as an increased risk for a subsequent relapse (odds ratio [OR] 3·28; 95% CI 1·22-9·18), more relapses (incidence rate ratio 1·77; 95% CI 0·96-3·25), fewer months until a relapse occurred (b -0·22; 95% CI -0·40 to -0·04), and more intense psychiatric care (OR 3·16; 95% CI 1·26-8·09) after the onset of psychosis.
  
  
  CONCLUSIONS
  Adverse effects associated with continued use of cannabis after the onset of a first episode of psychosis depend on the specific patterns of use. Possible interventions could focus on persuading cannabis-using patients with psychosis to reduce use or shift to less potent forms of cannabis.
  
  
  BACKGROUND
  National Institute for Health Research (NIHR).
authors: Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, UK.
authors: Tabea
authors: T
authors: Schoeler
authors: Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, UK.
authors: Natalia
authors: N
authors: Petros
authors: Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, UK.
authors: Marta
authors: M
authors: Di Forti
authors: Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, UK.
authors: Ewa
authors: E
authors: Klamerus
authors: Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, UK.
authors: Enrico
authors: E
authors: Foglia
authors: Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, UK.
authors: Olesya
authors: O
authors: Ajnakina
authors: Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, UK.
authors: Charlotte
authors: C
authors: Gayer-Anderson
authors: Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, UK.
authors: Marco
authors: M
authors: Colizzi
authors: Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, UK.
authors: Diego
authors: D
authors: Quattrone
authors: Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, UK.
authors: Irena
authors: I
authors: Behlke
authors: Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, UK.
authors: Sachin
authors: S
authors: Shetty
authors: Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, UK.
authors: Philip
authors: P
authors: McGuire
authors: Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, UK.
authors: Anthony S
authors: AS
authors: David
authors: Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, UK.
authors: Robin
authors: R
authors: Murray
authors: Institute of Psychiatry, Psychology and Neuroscience, King's College London, London, UK. Electronic address: sagnik.2.bhattacharyya@kcl.ac.uk.
authors: Sagnik
authors: S
authors: Bhattacharyya
mesh_terms: D000293:Adolescent; D000328:Adult; D017668:Age of Onset; D000368:Aged; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D008385:Marijuana Smoking / Q000009:adverse effects* / Q000453:epidemiology; D008875:Middle Aged; D011446:Prospective Studies; D011618:Psychotic Disorders / Q000453:epidemiology*; D012008:Recurrence; D012307:Risk Factors; D055815:Young Adult