Does change in cannabis use in established psychosis affect clinical outcome?
BACKGROUND
Cannabis use has been identified as a potent predictor of the earlier onset of psychosis, but meta-analysis has not indicated that it has a clear effect in established psychosis.


OBJECTIVE
To assess the association between cannabis and outcomes, including whether change in cannabis use affects symptoms and functioning, in a large sample of people with established nonaffective psychosis and comorbid substance misuse.


METHODS
One hundred and sixty participants whose substance use included cannabis were compared with other substance users (n = 167) on baseline demographic, clinical, and substance use variables. The cannabis using subgroup was examined prospectively with repeated measures of substance use and psychopathology at baseline, 12 months, and 24 months. We used generalized estimating equation models to estimate the effects of cannabis dose on subsequent clinical outcomes and whether change in cannabis use was associated with change in outcomes.


RESULTS
Cannabis users showed cross-sectional differences from other substances users but not in terms of positive symptoms. Second, cannabis dose was not associated with subsequent severity of positive symptoms and change in cannabis dose did not predict change in positive symptom severity, even when patients became abstinent. However, greater cannabis exposure was associated with worse functioning, albeit with a small effect size.


CONCLUSIONS
We did not find evidence of an association between cannabis dose and psychotic symptoms, although greater cannabis dose was associated with worse psychosocial functioning, albeit with small effect size. It would seem that within this population, not everyone will demonstrate durable symptomatic improvements from reducing cannabis.
School of Psychological Sciences, 2nd Floor Zochonis Building, University of Manchester, Oxford Road, Manchester M13 9PL, UK. christine.barrowclough@manchester.ac.uk
Christine
C
Barrowclough
Richard
R
Emsley
Emily
E
Eisner
Ruth
R
Beardmore
Til
T
Wykes
D000328:Adult; D063386:Cannabinoid Receptor Agonists / Q000009:adverse effects*; D004305:Dose-Response Relationship, Drug; D013759:Dronabinol / Q000009:adverse effects*; D005260:Female; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D008297:Male; D002189:Marijuana Abuse / Q000523:psychology*; D008385:Marijuana Smoking / Q000523:psychology*; D008875:Middle Aged; D011446:Prospective Studies; D011618:Psychotic Disorders / Q000523:psychology*; D019966:Substance-Related Disorders / Q000523:psychology
D063386:Cannabinoid Receptor Agonists; D013759:Dronabinol