Background Obesity is a major risk factor for osteoarthritis (OA) whilst there is some evidence that diabetes also increases risk. Metformin is a common oral treatment for those with diabetes. Objective The aim is to investigate whether metformin reduces the risk of OA. Methods This was a cohort study set within the Consultations in Primary Care Archive, with 3217 patients with type 2 diabetes. Patients at 13 general practices with recorded type 2 diabetes in the baseline period (2002–2003) and no prior record of OA were identified. Exposure was a prescription for metformin. Outcome was an OA record during follow up. Cox proportional hazard models with Gamma frailty term were fitted: adjusted for age, gender, deprivation, and comorbidity. Results There was no association between prescribed metformin treatment at baseline and OA (adjusted HR: 1.02, 95% CI: 0.91, 1.15). A similar non- significant association was found when allowing exposure status of prescription of metformin to vary over time.