The diabetes drug metformin -- commonly a first choice for controlling blood sugar in people with type 2 diabetes -- is sparking new interest as a cancer fighter.

A new study presented here at the American Association for Cancer Research (AACR) annual meeting shows that metformin (Fortamet, Glucophage, Glumetza, Riomet) may put the brakes on the growth of tumor cells in men with prostate cancer. Another study released in one of the association's journals suggests that it may extend the lives of people with pancreatic cancer.

But experts caution that the work is still preliminary and more study is needed before metformin can be recommended as a cancer treatment.

"These are still the early days," says Jose Baselga, MD, PhD, chief of hematology/oncology at Massachusetts General Hospital. "But there are strong signals of an anti-cancer effect."

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Metformin vs. Prostate Cancer

One new study involved 22 men with prostate cancer. They took metformin pills three times a day from the time they got their diagnosis to when they had their prostates removed, an average period of 41 days.

Researchers compared tissue from biopsies taken at diagnosis to tissue removed at the time of surgery and found that metformin slowed the growth of tumor cells by 32%.

Levels of insulin-like proteins in the blood also dropped.

Still unknown is whether men who take metformin are more likely to beat prostate cancer, says researcher Anthony M. Joshua, MBBS, PhD, a staff medical oncologist at Princess Margaret Hospital/University Health Network in Toronto.

But a growing body of evidence -- from lab, animal, and human studies -- suggests metformin mounts a multi-pronged attack against cancer, he tells WebMD. It lowers levels of insulin in the blood, and insulin contributes to the growth of cancer cells, Joshua says.

Metformin also shuts down the so-called mTOR pathway, which "is the cellular motor that makes cancer cells grow," he says. And it may slow down the metabolism of cancer cells, causing them to die off, Joshua says.

None of the men in the study had diabetes, so it is conceivable that men with diabetes and men without the condition both may benefit from metformin's cancer-fighting effects, he says.

None of the men experienced serious side effects from the drug in the study. Diarrhea, nausea, and gas are common side effects associated with its use in diabetes.
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"It's very early and the study is small," says Nancy Dawson, MD, a prostate cancer specialist at Georgetown Lombardi Comprehensive Cancer Center. "But's it’s fascinating to see such a slowing of cancer cell growth in such a short period of time."

Phillip Dennis, MD, PhD, of the National Cancer Institute, says, "The data suggest that if metformin is working, it is [attacking cancer cells in ways] that have not been identified. People should not start taking metformin for prostate cancer," he tells WebMD.

There will be about 240,000 new cases of prostate cancer in 2012, according to the American Cancer Society. About 13 million, or 11.8% of all men aged 20 years or older, have diabetes, according to the American Diabetes Association.