The researchers noted that metformin was also linked to better survival in diabetes patients diagnosed with ovarian and colorectal cancers.

“Our findings from this large cohort of postmenopausal women with long-term follow-up suggest that diabetes remains a risk factor for cancer and cancer-related death, and metformin therapy, compared to other diabetes medications, may have an important role in the prevention or management of diabetes-associated cancer,” said Dr Gong.

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While metformin use did not appear to affect invasive cancer risk overall, longer use was associated with a decreased risk for breast cancer (P– trend=.04). Furthermore, results revealed a trend of decreasing risk for cancer death with increasing duration of metformin use, but a trend of increasing risk for use other antidiabetes medications (P<.001).

“I think although more studies are necessary, such as clinical trials, to further address whether or how metformin is associated with decreased risk of certain cancers and cancer death, our findings from this large study may provide more evidence that postmenopausal women with diabetes and cancer may benefit from metformin therapy compared to other antidiabetes therapy,” Dr Gong told Endocrinology Advisor, highlighting the study’s strengths. “We had a long-term follow-up with a median follow-up of 15 years among these women. In this study, we collected detailed data on various factors, and collected information on medication use not only at baseline and updated during the entire follow-up period, limiting issues from potential bias.”

Nevertheless, the researchers noted that the analyses were limited by the small number of certain rarer cancers occurring in women with diabetes, especially when stratified by medication use. In addition, they reported that the findings may not be the same in premenopausal women or men.

Clinical Perspectives

Janet McGill, MD, professor of medicine and director of fellowship in endocrinology, diabetes, and metabolism at Washington University School of Medicine in St. Louis, Missouri, said this is a well done analysis and confirms prior reports, albeit with somewhat different findings relative to the strength of the association for each type of cancer and effects on mortality vs incidence.

Epidemiologic studies such as this study conducted by Gong and colleagues are hypothesis-generating, she noted. 

“These findings should be confirmed by both preclinical models of human disease and by randomized clinical trials. Whether metformin would benefit women without diabetes who develop breast or other cancer is not known and would be best served by clinical trials,” Dr McGill told Endocrinology Advisor. “Given the overall safety of metformin when used within prescribing guidelines for kidney function, the practical message for providers taking care of patients with diabetes may be to continue metformin therapy, and add on other antidiabetes therapies rather than replace metformin, for as long as possible. This is our current practice, which now has support from this Nurses Health Initiative Study.”