Metformin, a common diabetes drug that has been manufactured at low cost for years, has a long record of safety, effectiveness, and limited side effects in diabetics. In the mid-2000s an interesting discovery was made and published in the British Medical Journal: patients taking metformin for diabetes saw a significantly lowered risk for breast cancer (1). Further investigation into this phenomenon is ongoing at institution such as the Mayo Clinic and University of Chicago, focusing on several other types of cancer, as well (2).  As of May 2015, there were over 200 clinical trials testing the repurposing of Metformin listed on the website (3).

1. Evans J.M.M., Donnelly L.A., Emslie-Smith A.M., Alessi D.R., and Morris A.D. (2005). Research pointers: metformin and reduced risk of cancer in diabetic patients. British Medical Journal 330(7503): 1304-5. [Link here]
2. Sappa N. (2013). Old drug, new tricks: metformin, widely used for diabetes, takes a swipe at cancer. Science News 184(11): 18-21. [Link here]