To evaluate whether use of oral hypoglycemic agents is associated with an altered breast cancer risk in women.

Using the U.K.-based General Practice Research Database, we conducted a nested case-control analysis among 22,621 female users of oral antidiabetes drugs with type 2 diabetes. We evaluated whether they had an altered risk of breast cancer in relation to use of various types of oral hypoglycemic agents. Case and control patients with a recorded diagnosis of type 2 diabetes were matched on age, calendar time, and general practice, and the multivariate conditional logistic regression analyses were further adjusted for use of oral antidiabetes drugs, insulin, estrogens, smoking BMI, diabetes duration, and HbA1c (A1C).

We identified 305 case patients with a recorded incident diagnosis of breast cancer. The mean ± SD age was 67.5 ± 10.5 years at the time of the cancer diagnosis. Long-term use of ≥40 prescriptions (>5 years) of metformin, based on 17 exposed case patients and 120 exposed control patients, was associated with an adjusted odds ratio of 0.44 (95% CI 0.24–0.82) for developing breast cancer compared with no use of metformin. Neither short-term metformin use nor use of sulfonylureas or other antidiabetes drugs was associated with a materially altered risk for breast cancer.

A decreased risk of breast cancer was observed in female patients with type 2 diabetes using metformin on a long-term basis.