Insulin-treated patients with type 2 diabetes who use metformin have a lower risk of cancer compared with those not taking the drug, according to researchers.

Edoardo Mannucci, MD, the Diabetes Agency, Careggi Teaching Hospital in Florence, Italy, and colleagues conducted a study involving a cohort of 1,340 patients with type 2 diabetes treated with insulin. During a median follow-up of 75.9 months, cancer developed in 112 patients in the cohort (case subjects). The investigators compared these patients with 370 controls selected from the same cohort and matched for age, sex, and body mass index. A significantly lower proportion of case subjects than controls used metformin and sulfonylureas during follow-up.

After adjusting for potential confounders, metformin use was associated with a 54% decreased risk of cancer in men and a 45% decreased risk in women, whereas exposure to sulfonylureas was not associated with decreased risk of cancer in either men or women, Dr. Mannucci’s group reported in Diabetes Care (2011;34:129-131).

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The new findings corroborate previous research showing that metformin use by diabetics decreases their risk of cancer, they noted. This protective effect was evident even after adjusting for insulin doses, “suggesting that the protective action of metformin cannot be entirely attributed to its insulin-sparing effects,” the authors wrote.

The researchers concluded that “the reduction of cancer risk could be a further relevant reason for maintaining use of metformin in insulin-treated patients.”