The studies, published in the journal Cancer Prevention Research, support previous research that found diabetes patients receiving the drug metformin are less prone to developing cancer.
Metformin helps stabilize blood sugar by decreasing the liver's glucose output and increasing the sugar's use by muscle tissue.
Scott Lippman, an oncologist at the M.D. Anderson Cancer Center in Houston, estimates more than 40 million metformin prescriptions have been filled in the United States.
"It's been around for a while," he says.
In one study, metformin reduced the appearance of lesions that can be precursors to bowel cancer. In another study the drug reduced lung cancer tumors in mice.
Metformin is derived from plants and was used in medieval times to treat frequent urination. Modern scientists became interested in the drug in the 1920s for its ability lower blood sugar.
A long history of use with few side effects may help metformin as it faces regulatory hurdles to become a cancer fighter, Lippman says.
"A lot is known about the safety of this," he says. "From a research perspective, this is extremely exciting. The next step will be to take it to a clinical trial."