The largest study of its kind ever conducted has found that women who take estrogen after menopause run an increased risk of developing breast cancer.
But experts said the findings did not mean that post-menopausal women should stop taking estrogen. The benefits of the drug are great, they said, and the increased risk of breast cancer is relatively small.
The researchers, led by Dr. Graham A. Colditz of Brigham and Women's Hospital in Boston, also found that a year after women stop taking the female sex hormones, the additional risk subsides.
Cancer researchers have spent years trying to clarify the risks women may face when they take estrogen. The hormone can prevent a loss of calcium from bones that occurs when the body's estrogen production falls at menopause. Estrogen may also prevent heart disease, the leading killer of older women.
Studies have shown that estrogen can cause cancer of the uterus, a relatively rare and much less dangerous cancer. Far more troubling were studies suggesting that taking estrogen might also contribute to breast cancer, a major cause of death.
The researchers followed 121,700 female nurses for 10 years, recording whether the women took estrogen after menopause and for how long.
They found that women taking estrogen were 30 percent to 40 percent more likely to develop breast cancer than those who did not take the hormone. But this risk is only about half the risk a woman faces if her mother had breast cancer.