Background: Patients who experience the onset of psychotic illness with a comorbid diagnosis of cannabis dependence experience poor clinical outcomes. Few studies have identified interventions that reduce cannabis use and improve clinical outcome in this population.

Aims: We undertook a multi-center, randomized controlled trial of a group psychological intervention for psychosis with comorbid cannabis dependence to determine whether there was any impact on cannabis use symptoms, global functioning, insight, attitudes to treatment and subjective quality of life.

Method: Across three centers, we compared a group psychological intervention, based on cognitive behavioral therapy and motivational interviewing, with treatment as usual among patients experiencing their first psychotic episode or early in the course of psychotic illness. Substance misuse and indices of clinical outcome were assessed at baseline, 3 months and 1 year.

Results: At 3 month and 1 year follow-ups, there was no evidence for an intervention effect on cannabis use, symptoms, global functioning insight or attitude to treatment. However, the intervention improved subjective quality of life at 3 months and this effect was sustained at 1 year.

Conclusions: Over the early phase of psychotic illness, group psychological interventions for those with comorbid cannabis dependence improved subjective quality of life. However, this was not associated with reduction in use of cannabis or improvement in clinical outcomes.