Background Opioid substitution treatment (OST) reduces the harm of injecting and opioid dependence. The SCID feasibility trial explored the processes of conducting a randomized control trial (RCT) with people who inject drugs (PWID) in a low-threshold agency. Feasibility of the intervention investigated whether offering PWID immediate access to OST via specialist primary care increased numbers in OSTat 3 months, compared with offering advice and case management. Methods Un-blinded RCTwas conducted at Bristol Drugs Project needle exchange. A total of 311 individuals were eligible and 100 consented to participate. Trial process outcomes involved exploring OST status at 3 months; secondary outcomes were substance use and health-related quality of life measures. Results Follow-up was 86%. At 3 months, 51% intervention and 47% of control participants were in OST (OR of success of intervention 1.17 (0.54-2.57)). Opioid use reduced by 79 and 73%, respectively (OR of intervention success 1.38 (0.5-3.7)). Physical and mental health improved but there was little differences between groups. Conclusions The feasibility of conducting the trial was a success, but there was insufficient evidence of an effect compared with intensive case management. Further development and evaluation of case management approaches in low-threshold agencies is warranted.