Breast Cancer Coverage from Every Angle

Are Type 2 Diabetes and Metformin Use Associated With Breast Cancer Risk?

By: Vanessa A. Carter, BS
Posted: Friday, April 9, 2021

Dale P. Sandler, PhD, of the National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina, and colleagues performed a study to examine the association between type 2 diabetes and metformin use and breast cancer risk. Their research, published in the Annals of Oncology, concluded that correlations between these three factors may exist.

“These findings suggest that having type 2 diabetes may increase the risk of developing breast cancer, but that taking metformin may protect against developing estrogen receptor–positive breast cancer…,” Dr. Sandler stated in a European Society for Medical Oncology press release. Compared with nondiabetic women, “the risk of developing estrogen receptor–positive breast cancer was reduced by 38% among women with type 2 diabetes who had used metformin for 10 years or more.”

The study authors focused on 44,541 individuals with sisters or half-sisters who had a history of breast cancer, but participants themselves did not. Information including incident and prevalent type 2 diabetes, the use of antidiabetic medications, and covariates was retrieved from baseline and questionnaires at follow-up. Incident and prevalent breast cancers were obtained from medical records.

Of the total participants, 3,227 and 2,289 women had prevalent and incident type 2 diabetes, respectively; 61% of these patients were treated with metformin. At the median follow-up of 8.6 years, it was reported that 2,678 women were diagnosed with breast cancer at least 1 year after being enrolled. Although there was no apparent association between overall risk of breast cancer and type 2 diabetes, there appeared to be an increased risk of triple-negative breast cancer.

Additionally, the use of metformin for treatment of type 2 diabetes did not seem to correlate with overall breast cancer risk, but it was associated with an increased risk of triple-negative and estrogen receptor–negative breast cancers. Use of metformin was also related to a decreased risk of estrogen receptor–positive breast cancer, with longer durations of treatment inversely seeming to affect cancer risk.

Disclosure: The study authors reported no conflicts of interest.

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