Relation between diabetes, metformin treatment and the occurrence of malignancies in a Belgian primary care setting

Background: Associations between type 2 diabetic patients and a higher risk of developing cancer have been reported worldwide. Recently, a protective effect of metformin has been described. Aim: To examine in the Belgian primary care population the relation between presence of type 2 diabetes with and without metformin treatment and the occurrence of malignancies. Design of study: Retrospective cohort study, based on the Intego database, an ongoing Belgian general practice-based morbidity registry, covering 90 general practitioners and including about 1.5 million patient-years between 1994 and 2008. Method: Cox proportional hazard analysis comparing emergence of malignancy in patients with and without type 2 diabetes, and among patients with diabetes comparing emergence of malignancy in those treated with various antidiabetic drugs. Results: Malignancies occurred more in type 2 diabetic patients compared to non-diabetic controls (HR = 1.84; 95% CI = 1.51-2.24), adjusted for age, gender and weight. Treatment with both metformin and 'other' antidiabetic agents was related to decreased cancer risk (HR = 0.24 and 0.22) compared to diet only in men but not in women. Conclusion: In this Belgian primary care setting, diabetic patients have higher cancer prevalences than non-diabetic patients. Moreover, in diabetic men, not only metformin but also other antidiabetic agents were associated with lower cancer risks.