Disclosed herein is a method for treating a tumor in a subject in need thereof comprising administering an enhancing amount of metformin and a reduced amount of one or more chemotherapeutic agents. One example of an enhancing amount of metformin is about 250 mg/day. Also disclosed is a method for preventing cancer or delaying the recurrence of cancer in a subject comprising administering an effective amount of metformin to the subject. In one example of such a method, the amount of metformin is about 75 mg/day. Also disclosed is a composition comprising an enhancing amount of metformin, and a reduced amount of one or more chemotherapeutic agents and a pharmaceutically acceptable carrier. Kits comprising metformin and one or more chemotherapeutic agents are also disclosed.
