Metformin-takers tended to be healthier in all sorts of ways. They lived longer and had fewer cardiovascular events, and in at least some studies they were less likely to suffer from dementia and Alzheimer's. Most surprising of all, they seemed to get cancer far less frequently—as much as 25 to 40 percent less than diabetics taking two other popular medications. When they did get cancer, they tended to outlive diabetics with cancer who were taking other medications.

Lewis Cantley, the director of the Cancer Center at Weill Cornell Medicine, once put it, "Metformin may have already saved more people from cancer deaths than any drug in history." Nobel laureate James Watson (of DNA-structure fame), who takes metformin off-label for cancer prevention, once suggested that the drug appeared to be "our only real clue into the business" of fighting the disease.