The risk of hepatitis C virus (HCV) reinfection due to continued injecting risk behaviours might remain a barrier to HCV treatment among people who inject drugs. We aimed to evaluate changes in risk behaviours during and following HCV treatment among people with ongoing injecting drug use or receiving opioid substitution treatment (OST).ACTIVATE was an international multicentre clinical trial conducted between 2012 and 2014. Participants with HCV genotypes 2/3 infection were treated with peg-interferon/ribavirin for 12 or 24 weeks and completed a self-administered behavioural questionnaire at each study visit. The impact of time in treatment and follow-up on longitudinally measured recent (past month) behavioural outcomes was evaluated using generalized estimating equations.Among 93 enrolled participants (83% male, median age 41 years), 55 (59%) had injected in the past month. Any injecting drug use decreased during HCV treatment and follow-up (OR 0.89 per incremental study visit; 95% CI 0.83-0.95). No significant changes were found in ≥daily injecting (OR 0.98; 95% CI 0.89-1.07), use of non-sterile needles (OR 0.94; 95% CI 0.79-1.12), sharing of injecting paraphernalia (OR 0.87; 95% CI 0.70-1.07) or non-injecting drug use (OR 1.01; 95% CI 0.92-1.10). Hazardous alcohol use decreased throughout (OR 0.56; 95% CI 0.40-0.77) and OST increased between enrolment and end of treatment (OR 1.48; 95% CI 1.07-2.04).Recent injecting drug use and hazardous alcohol use decreased, while OST increased during and following HCV treatment among participants with ongoing injecting drug use. These findings support further expansion of HCV care among PWID.