Does metformin lower your risk of cancer or dementia?

Metformin has for years been associated with a lower risk of cancer in people with diabetes. Observational research shows that metformin users develop cancer less often and also die from it less often. The drug is inexpensive and has a favourable safety profile. However, this evidence is based on observational studies, not large randomised trials. That means other factors could explain the difference -- for example, that healthier diabetes patients are more likely to be prescribed metformin.

The picture varies by cancer type. For breast cancer, the risk was slightly lower in metformin users than in non-users, but that difference was not statistically convincing enough to draw any conclusions from. For prostate cancer, a large analysis of more than 52,000 cases found no protective effect at all. In short: the overall signal is positive, but for specific cancer types the supporting evidence is weak or absent.

Exactly how metformin might work against cancer is not yet well understood. The drug influences the energy metabolism of cells, both throughout the body and directly on cancer cells. Researchers are already studying these pathways intensively, but how laboratory results translate into clinical benefits in humans remains uncertain.

On the topic of dementia, nothing can be said on the basis of the available studies. There are simply no usable data available to support any statement about metformin and dementia risk. Anyone looking for information on this is best advised to discuss it with a doctor.