Iris Romero, MD, MS, associate professor of obstetrics and gynecology at The University of Chicago Medicine in Illinois, said this paper makes an important contribution to a longstanding question regarding the relationship between diabetes and cancer, and sheds light on a new question of whether or not a common diabetes medication — metformin — protects against cancer. 

“The study had unique strengths that other studies that have tried to answer these questions did not have, including a large samples size, long follow-up, and detailed information about medication use,” Dr Romero told Endocrinology Advisor.

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“However, even with a dataset as large as the WHI and this rigorous statistical analysis, several ongoing questions remain unanswered. First, are there specific cancers where the effect of diabetes and metformin are more important? Because of the limited number of each individual cancer types, we would not be able to use these findings to advise patients about risk or survival of specific cancer. Second, what would be the effect of metformin in patients without diabetes?”

Ultimately, she noted, the role of metformin in cancer can only be answered by prospective clinical trials in patients without diabetes.  These trials are underway in several cancers including breast cancer and a trial in ovarian cancer, which Dr Romero is helping conduct.

Cynthia Stuenkel, MD, clinical professor of medicine at the University of California, San Diego in LaJolla, California, said the study was “interesting” and “well powered.” However, she noted that it was limited by its observational design.

“Right now it would seem reasonable to counsel women with diabetes who were already taking or about to take metformin that some studies suggest that cancer mortality might be reduced in women taking metformin compared to other diabetes drugs, but this finding was preliminary and had not been proven in properly conducted randomized controlled trials,” she told Endocrinology Advisor. “It would be premature, based on this observational data, to recommend metformin for cancer prevention or mortality benefit.”

UPDATE: This article has been updated to correct Dr Stuenkel’s comments. A previous version misquoted Dr Stuenkel as calling the study “excellent.” The updated version more accurately reflects Dr Stuenkel’s statements.


  1. Gong Z, Aragaki AK, Chlebowski RT, et al. Diabetes, metformin and incidence of and death from invasive cancer in postmenopausal women: Results from the women’s health initiative. Int J Cancer. 2016;138:1915–1927. doi: 10.1002/ijc.29944.