Type 2 diabetes mellitus (T2DM) imposes a substantial burden on New Zealanders, particularly Maori and Pacific Peoples. Metformin is the first-line medication for the treatment of T2DM, but overseas research has found that many patients have suboptimal metformin adherence and persistence. Persistence refers to whether a person stays on a therapy, while adherence refers to whether a person takes a medication according to the prescribed schedule. Suboptimal metformin adherence and persistence are important because they are associated with a higher risk of diabetes complications. We will use routinely collected health data to (a) describe the patterns of metformin adherence and persistence in New Zealanders with T2DM, and (b) investigate the predictors and consequences of suboptimal adherence and persistence. We will also interview people with T2DM to get their views on what helps, and hinders, good metformin adherence and persistence. The findings will contribute to improved outcomes for people prescribed metformin.