Researchers found that women with diabetes who took the medication had a 25-percent lower risk of developing breast cancer over more than a decade of follow-up.

But like all previous studies of metformin's suspected anti-cancer effects, the new findings fall short of proving the compound can stave off the big C.

"This is an area of great excitement," said Dr. Pamela Goodwin, who wrote an editorial about the study, published in the Journal of Clinical Oncology.

"The evidence is coming together that metformin may actually have a clinically relevant effect, but none of this is good enough to change clinical practice just yet," added Goodwin, a breast cancer expert at Mount Sinai Hospital in Toronto, Canada.

Metformin (also sold under the brand name Glucophage) has been on the market for many years and is generally considered safe, although five to 10 percent of patients experience side effects like nausea and bloating. Because its original patent has expired, a month's worth of pills can be bought for less than $10.

It is used by millions of type 2 diabetics every day to help control their blood sugar. Intriguingly, metformin also shrinks lung and breast tumors in mice, and several reports show people taking it for diabetes appear to develop cancer less often.

That's why scientists are hopeful it might someday be used to prevent cancer in smokers and others at high risk of the disease.

The new study used data from about 68,000 postmenopausal women who took part in the U.S. government-funded Women's Health Initiative clinical trials.

Over nearly 12 years of observation, there were more than 3,200 new cases of breast cancer among the women.

Every year, 0.42 percent of women without diabetes developed breast cancer, compared to 0.40 percent of diabetics on metformin and 0.47 percent of diabetics taking other drugs.

After taking into account risk factors for breast cancer, the gap between women without diabetes and diabetics on drugs other than metformin vanished. But diabetics on metformin turned out to have a 25-percent lower cancer risk than their diabetes-free peers.

Goodwin said the new study is the best of its kind so far, but nonetheless relies on observations instead of an actual experiment in which women are randomly selected to take metformin or not.

"All of these observational studies, what they do is they generate hypotheses," she told Reuters Health. "I think we need to be cautious about taking these observations and applying them to people without diabetes."

Dr. Rowan Chlebowski, who led the study, said his results don't necessarily mean diabetics should switch to metformin if they are not already taking it.

"The significance is identifying potential other uses for drugs we have great safety information on, as opposed to new drugs," Chlebowski, of the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center in Torrance, told Reuters Health.

Goodwin agreed.

"You should have whatever treatment is best for your diabetes," she said. "Most diabetics are already on metformin, but the key is they should get good blood sugar control."

Goodwin and her colleagues are currently running a $25 million trial to test whether metformin can help ward off new tumors in women getting breast cancer treatment. The results are expected in three to four years.

Goodwin said nobody has yet begun a study to see if the drug can prevent healthy people from getting cancer in the first place.

"At a societal level, we have to develop mechanisms to take the inexpensive generic drugs that seem to have anti-cancer effects and study them," she added.

According to the American Cancer Society, one in eight women will develop breast cancer at some point, although less than three percent of all women will die of the disease.

SOURCE: bit.ly/L86p7l Journal of Clinical Oncology, online June 11, 2012.

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