An increasing wealth of knowledge in oncology and diabetes points to metformin, the most widely used oral anti-diabetic worldwide, as a potential anticancer drug. Metformin has not only been shown to reduce the risk of developing cancer, but recent studies have also highlighted the impact that this drug might have on cancer aggressiveness, progression and survival. Several categories of confounding factors, some of them not taken into account in current clinical studies, may lead to an incorrect estimation of the true effect that metformin might have in cancer patients. It is important to assess whether the insulin lowering effects of metformin may constitute the main antineoplastic mechanism of action or if this may be due to an underlying direct effect. Such reasons warrant that new prospective research should take into account patient insulin levels, the duration and doses of metformin treatment and also include a cohort of non-diabetic patients for comparison. Measurement of the optimal antineoplastic dosage of metformin and proper quantification of established confounding factors will likely lead to a better understanding and estimation of metformin’s true anti-cancer effects, ultimately resulting in a better clinical management for diabetic patients with cancer.
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