BACKGROUND: Several studies have indicated that type 2 diabetes mellitus is more common among schizophrenic patients than in the general population. In this study, we investigated whether the use of antipsychotic drugs in patients with diabetes leas to worsening of glycemic control. METHOD: In this cohort study, patients with newly diagnosed type 2 diabetes were selected from the PHARMO Record Linkage System, which comprises pharmacy records for all 320000 residents of 6 Dutch cities. In total, weidentified 2585 patients with incident cases of type 2 diabetes who began treatment with oral hypoglycemic agents between 1991 and 1997 and had a medication history of at least 2 years after diagnosis of diabetes. A change in treatment from oral hypoglycemic agents alone to insulin therapy (with or without continuation of oral hypoglycemic agents) was considered a proxy for deterioration of beta-cell function. We compared the incidence of initiation of insulin therapy between users and nonusers of antipsychotic drugs by performing a Cox proportional hazards model analysis. RESULTS: We found an increased risk for initiation of insulin therapy at 2 years after diagnosis of diabetes in users of antipsychotics compared with nonusers; the relative hazard (hazard ratio) was 2.0 (95% CI = 1.2 to 3.3), which did not change after adjustment for potential confounders. The risk decreased in the years after diagnosis of diabetes. CONCLUSION: It seems that use of antipsychotics by patients with type 2 diabetes mellitus is associated with initiation of insulin therapy (i.e., “secondary failure”), especially in the first 2 years of the disease.