The hormone estrogen that millions of American women take to offset menopause symptoms may cause nearly 5,000 new cases of breast cancer each year, government researchers reported.

An analysis of 16 studies found women who took the hormone for at least 15 years had about a 30 percent greater risk of breast cancer.About 4,708 new breast cancer cases and 1,468 deaths may result annually, researchers reported.

"Although the overall benefit of estrogen replacement after menopause may outweigh the risks for most women, our analysis supports a small but statistically significant increase in breast cancer risk due to long-term estrogen use," the researchers said Tuesday.