"Type 2 diabetes increases the risk for several types of cancer," said lead author Diego Espinoza-Peralta, MD, an endocrinologist with Mexico's National Institute of Medical Sciences and Nutrition (Instituto Nacional de Ciencias Medicas y Nutricion) in Mexico City.

"Our findings suggest that the regular use of metformin—a low-cost medication— reduces this risk, compared with not taking metformin."

Espinoza-Peralta and his colleagues conducted a systematic review and meta-analysis, or combined statistical analysis, of reported studies that evaluated cancer risk in patients with Type 2 diabetes. They analyzed seven relevant studies that included more than 32,400 Type 2 diabetic patients who had no other known condition that increased their cancer risk.

The investigators found that the odds of getting any type of cancer was 0.62 times less—an estimated 38 percent relative risk reduction—with daily continuous use of metformin than for those with no exposure to metformin.

"This risk reduction with metformin use extended to certain types of cancers, specifically colon and breast cancer," Espinoza-Peralta said.

Colorectal cancer and breast cancer are among the cancers that studies have found to occur more often in people with Type 2 diabetes. There was no risk reduction, however, in pancreatic cancer, another type of cancer for which people with Type 2 diabetes are at increased risk, the authors reported.

Metformin, which is the standard recommended initial treatment of Type 2 diabetes, may protect against cancer because it regulates activity of an enzyme that suppresses cell growth, according to Espinoza-Peralta.

"There is growing evidence that metformin brings more benefits to diabetic patients beyond glucose control," he said.

Source: The Endocrine Society 

Metformin's effect on breast cancer