Certain types of cancer are more common in people with diabetes than in those without, but a recent analysis has found that cancer patients with diabetes who were on metformin monotherapy have a significantly reduced overall mortality compared with cancer patients without diabetes. Previous research has shown similar results.

The study evaluated 8,392 people with type 2 diabetes and 104,016 without diabetes who developed a first tumor. The diabetes patients who had been on metformin monotherapy for 90 days before cancer diagnosis had significantly reduced overall mortality (hazard ratio [HR] 0.85) compared with cancer patients without diabetes (Diabetes Care. 2012;35:299-304). Among people with diabetes, significantly increased mortality was seen in those treated with a sulfonylurea alone (HR 1.48) or with insulin alone (HR1.33) compared with persons taking metformin alone.

Another recent investigation yielded the unexpected finding that exposure to metformin reduces cellular mutation rate and the accumulation of DNA damage. As Dr. Michael N. Pollak, professor in the departments of medicine and oncology at McGill University in Montreal, Canada, and colleagues reported in Cancer Prevention Research, such mutations are known to be directly involved in carcinogenesis, but lowering cancer risk by inhibiting the mutation rate had never been shown to be feasible. The group’s work suggests that metformin reduces DNA damage by reducing levels of reactive oxygen species, which are known to be DNA-damaging agents produced when cells generate energy from nutrients.

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“It is remarkable that metformin—an inexpensive, off-patent, safe, and widely used drug—has several biological actions that may result in reduced cancer risk,” noted Pollak in a statement describing his team’s findings.

In the Diabetes Care study, investigators also confirmed that cancer mortality is higher in those with diabetes than in those without (HR 1.09), but the relatively small difference may reflect reduced survival due to diabetes rather than a worse outcome from the cancer. In adjusted analyses, the presence of diabetes significantly increased mortality in prostate cancer (HR 1.19) and in breast cancer (HR 1.32). However, diabetes was associated with improved lung cancer survival compared with patients without diabetes (HR 0.84).