title: Proportion of patients in south London with first-episode psychosis attributable to use of high potency cannabis: a case-control study.
abstract: BACKGROUND
  The risk of individuals having adverse effects from drug use (eg, alcohol) generally depends on the frequency of use and potency of the drug used. We aimed to investigate how frequent use of skunk-like (high-potency) cannabis in south London affected the association between cannabis and psychotic disorders.
  
  
  METHODS
  We applied adjusted logistic regression models to data from patients aged 18-65 years presenting to South London and Maudsley NHS Foundation Trust with first-episode psychosis and population controls recruited from the same area of south London (UK) to estimate the effect of the frequency of use, and type of cannabis used on the risk of psychotic disorders. We then calculated the proportion of new cases of psychosis attributable to different types of cannabis use in south London.
  
  
  RESULTS
  Between May 1, 2005, and May 31, 2011, we obtained data from 410 patients with first-episode psychosis and 370 population controls. The risk of individuals having a psychotic disorder showed a roughly three-times increase in users of skunk-like cannabis compared with those who never used cannabis (adjusted odds ratio [OR] 2·92, 95% CI 1·52-3·45, p=0·001). Use of skunk-like cannabis every day conferred the highest risk of psychotic disorders compared with no use of cannabis (adjusted OR 5·4, 95% CI 2·81-11·31, p=0·002). The population attributable fraction of first-episode psychosis for skunk use for our geographical area was 24% (95% CI 17-31), possibly because of the high prevalence of use of high-potency cannabis (218 [53%] of 410 patients) in our study.
  
  
  CONCLUSIONS
  The ready availability of high potency cannabis in south London might have resulted in a greater proportion of first onset psychosis cases being attributed to cannabis use than in previous studies.
  
  
  BACKGROUND
  UK National Institute of Health Research (NIHR) Specialist Biomedical Research Centre for Mental Health, SLaM and the Institute of Psychiatry at King's College London, Psychiatry Research Trust, Maudsley Charity Research Fund, and th European Community's Seventh Framework Program grant (agreement No. HEALTH-F2-2009-241909 [Project EU-GEI]).
authors.affiliation: Department of Psychosis Studies, Institute of Psychiatry, Kings College London, London, UK. Electronic address: marta.diforti@kcl.ac.uk.
authors.forename: Marta
authors.initials: M
authors.lastname: Di Forti
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 Psychosis Studies, Institute of Psychiatry, Kings College London, London, UK.
authors.forename: Elena
authors.initials: E
authors.lastname: Carra
authors.affiliation: Department of Psychosis Studies, Institute of Psychiatry, Kings College London, London, UK.
authors.forename: Sara
authors.initials: S
authors.lastname: Fraietta
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 Psychosis Studies, Institute of Psychiatry, Kings College London, London, UK.
authors.forename: Matteo
authors.initials: M
authors.lastname: Bonomo
authors.affiliation: Department of Psychosis Studies, Institute of Psychiatry, Kings College London, London, UK.
authors.forename: Francesca
authors.initials: F
authors.lastname: Bianconi
authors.affiliation: Department of Psychosis Studies, Institute of Psychiatry, Kings College London, London, UK.
authors.forename: Poonam
authors.initials: P
authors.lastname: Gardner-Sood
authors.affiliation: Department of Psychosis Studies, Institute of Psychiatry, Kings College London, London, UK.
authors.forename: Jennifer
authors.initials: J
authors.lastname: O'Connor
authors.affiliation: Department of Psychiatry, Mount Sinai School of Medicine, New York, NY, USA.
authors.forename: Manuela
authors.initials: M
authors.lastname: Russo
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: Tiago Reis
authors.initials: TR
authors.lastname: Marques
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: Paola
authors.initials: P
authors.lastname: Dazzan
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: Anthony S
authors.initials: AS
authors.lastname: David
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: Zerrin
authors.initials: Z
authors.lastname: Atakan
authors.affiliation: Department of Psychosis Studies, Institute of Psychiatry, Kings College London, London, UK.
authors.forename: Conrad
authors.initials: C
authors.lastname: Iyegbe
authors.affiliation: Department of Neuroscience, Institute of Psychiatry, Kings College London, London, UK.
authors.forename: John
authors.initials: J
authors.lastname: Powell
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 Addiction, Institute of Psychiatry, Kings College London, London, UK.
authors.forename: Michael
authors.initials: M
authors.lastname: Lynskey
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: D000293:Adolescent; D000328:Adult; D000368:Aged; D002188:Cannabis / Q000009:adverse effects*; D016022:Case-Control Studies; D005260:Female; D006801:Humans; D008131:London / Q000453:epidemiology; D008297:Male; D008875:Middle Aged; D011618:Psychotic Disorders / Q000453:epidemiology*; D012307:Risk Factors; D055815:Young Adult