Substance misuse among people with severe mental disorders is very common and has been associated with a number of adverse outcomes. Most trials among people with severe mental disorders have reported general substance misuse outcomes and have argued that longer treatments are likely to be more effective. Such studies have not specifically compared intervention effectiveness for alcohol vs. cannabis misuse. The present paper reports findings from a synthesis of datasets from three randomized controlled trials (N=248) comparing brief (1-session advice or motivational interviewing, MI) and extended (10 sessions MI/cognitive-behaviour therapy; CBT) interventions for substance misuse among people with psychosis or major depression and outcomes are reported separately for alcohol and cannabis use. Whilst there were comparable rates of reduction in alcohol consumption for the BI and intensive MI/CBT, BI tended to be associated with less reduction in cannabis use. A stepped care approach for substance misuse in people with severe mental disorders may help to improve cannabis outcomes, including monitoring of both mental health and substance misuse domains, with interventions stepped up or down according to treatment response. Future studies should evaluate the effectiveness of stepped care approaches.