[Which anti-tumour benefits to be expected from metformin?]

With the better management of cardiovascular risk factors, cancer plays an increasing role in the causes of death among patients with type 2 diabetes. Numerous epidemiological cohort and case-control studies showed that type 2 diabetes is a risk factor for cancer and that metformin therapy is associated with a significant reduction in the incidence of cancer and cancer-related death when compared to other glucose-lowering agents (sulfonylureas, insulin). Such beneficial effect is observed almost whatever the type of cancer, but seems to be more prominent in case of gastrointestinal and breast cancers. Several studies showed a significant relationship between the amplitude of the protection against cancer, on the one hand, and the daily dose of metformin and the duration of exposure, on the other hand. In general, the protective effect was more evident in observational cohort studies (however, more exposed to bias due to confounding factors) than in case-control studies. Several meta-analyses recently confirmed that metformin therapy reduces the incidence of cancers and cancer-related mortality. However, the results of the rather rare controlled clinical trials available are not conclusive, but none of them was performed with the objective to specifically assess cancer risk. Considering all promising clinical information in patients with type 2 diabetes, further clinical trials are currently ongoing with the aim of assessing the role of metformin in oncology, independently of the presence of diabetes.