Published on March 17, 2021
Metformin May Reduce Risk of Developing Breast Cancer
A medication used to control high blood sugar in patients with type 2 diabetes may reduce the risk of developing estrogen-positive (ER-positive) breast cancer in women.
In a study published in Annals of Oncology, the risk for ER-positive breast cancer was 38% lower among women with type 2 diabetes who had used the drug metformin for 10 years or more when compared to non-diabetic women. ER-positive breast cancer accounts for about 80% of breast cancer cases diagnosed in the United States.
“We also found that having type 2 diabetes for 15 years or more seemed to be associated with a 39% reduced risk of ER-positive breast cancer, and we think this is most likely to be because of long-term use of metformin,” said study author Dr. Dale Sandler, chief of the epidemiology branch at the U.S. National Institute of Environmental Health Sciences (NIEHS) in Research Triangle Park, N.C., in a press release.
The study included more than 44,000 women, ranging from 35 to 74 years of age, who participated in the Sister Study, which was conducted from 2003 to 2009. While they had never been diagnosed with breast cancer before the study started, the participants were sisters or half-sisters of women who had been diagnosed with breast cancer. Participants completed questionnaires every three years.
What About Other Types of Breast Cancer?
Metformin did not appear to reduce the risk of developing other breast cancers. In fact, it was associated with a 25% increased risk of developing ER-negative breast cancer and a 74% increased risk of developing triple-negative breast cancer, when compared to non-diabetic women.
“We can't say for sure if the increased risk of triple-negative breast cancer is because metformin doesn't protect women against the negative effects of having type 2 diabetes or because metformin use can cause triple-negative breast cancer,” Dr. Sandler said. “Since there are no mechanistic data supporting a causal effect of metformin, the former interpretation seems more likely."
Because of the small number of women with type 2 diabetes who were diagnosed with breast cancer (only 25 with triple-negative during the study), more research is needed.
Metformin: A Miracle Drug?
Scientists have hailed metformin as a miracle drug. It is commonly prescribed to treat women with polycystic ovary syndrome (PCOS). An April 2019 study published in Annals of Internal Medicine found that metformin could help people lose weight and keep it off. Researchers are also studying the drug’s effectiveness in extending life span and health span — the proportion of a person’s life that they spend in good health.
Of the women who developed type 2 diabetes after joining the study, those who were treated with medications other than metformin had twice the risk of developing any type of breast cancer compared to non-diabetic women, and 2.6 times the risk of developing ER-positive breast cancer, according to the release.
In diabetes, metformin reduces blood sugar by decreasing the amount of glucose produced in the liver. How the drug works in breast cancer is less clear. Researchers speculate that metformin may inhibit estrogen receptors that play a role in the development and progression of breast cancer. It also may slow cancer cell growth by activating the adenosine monophosphate protein kinase (AMPK). AMPK inhibits a pathway involved in the spread of cancer cells.
I Don’t Have Type 2 Diabetes, Should I Start Taking Metformin?
This does not mean that women without diabetes should take metformin to reduce breast cancer risk, said Dr. Pamela Goodwin, a professor of medicine at the University of Toronto and co-author of an editorial that accompanied the study.
She told HealthDay that losing weight and engaging in regular physical activity can help lower diabetes risk. This may have spillover benefits for breast cancer risk as well.
Dr. Goodwin’s editorial noted that the evidence linking the potential effects of type 2 diabetes treatment (including metformin) to breast cancer risk is inconsistent, mainly because of the small numbers of cases in diabetic subjects (overall and for specific subtypes), the differences in the characteristics of the participants (i.e., age/menopausal status and race) and the variability in diabetes treatments.
"If you have type 2 diabetes, get properly treated and in many cases, this means taking metformin,” Dr. Goodwin told HealthDay. “Make sure to get regular mammograms, too.”
- A Prospective Study of Type 2 Diabetes, Metformin Use, and Risk of Breast Cancer
- Metformin May Affect Risk of Breast Cancer in Women With Type 2 Diabetes
- Long-Term Weight Loss With Metformin or Lifestyle Intervention in the Diabetes Prevention Program Outcomes Study
- Type 2 Diabetes Drug Metformin Could Help Prevent Some Breast Cancers
- Diabetes, Metformin and Breast Cancer: A Tangled Web
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