Women with Type 2 diabetes may be more likely to develop breast cancer, but according to one study taking the diabetes drug metformin appears to reduce their risk for the most common type. Compared to women without diabetes, risk for estrogen-positive breast cancer was 38 percent lower among women with Type 2 diabetes who had used metformin for 10 years or more.

Metformin did not protect against estrogen-negative breast cancer and may increase risk for triple-negative breast cancers.

The study included more than 44,000 women ages 35 to 74 who were followed for more than eight years, completing questionnaires every three years. Though they had never been diagnosed with breast cancer when the study began, they were sisters or half-sisters of women diagnosed with breast cancer.

While women with Type 2 diabetes who took metformin were protected against estrogen-positive breast cancer, they had a 74 percent higher risk of developing triple-negative breast cancer.