To explore whether reducing substance misuse through a brief motivational intervention also reduces aggression and HIV risk behaviours. Participants were enrolled in a randomized controlled trial in primary care if they screened positive for substance misuse. Substance misuse was assessed using the Alcohol, Smoking and Substance Involvement Screening Test; aggression, using a modified version of the Explicit Aggression Scale; and HIV risk, through a count of common risk behaviours. The intervention was received on the day of the baseline interview, with a 3-month follow-up. Participants who received the intervention were significantly more likely to reduce their alcohol use than those who did not; no effect was identified for other substances. In addition, participants who reduced substance misuse (whether as an effect of the intervention or not) also reduced aggression but not HIV risk behaviours. Reducing substance misuse through any means reduces aggression; other interventions are needed for HIV risk reduction.