Chicago ? Women who take estrogen-only pills for at least 15 years run a markedly higher risk of developing breast cancer, according to a study of nearly 29,000 nurses. But no increased danger was found among those who took the hormone for less than 10 years.

Researchers said the findings should be reassuring for women who want to use estrogen for a short time to relieve menopausal symptoms.

Hormone supplements were once thought to help postmenopausal women postpone age-related ills. But the government’s Women’s Health Initiative study in 2002 contradicted those beliefs for estrogen-progestin supplements, finding an increased risk of breast cancer, strokes and heart attacks. That led millions of American women to stop taking supplements.

The new findings came from the less-rigorous but longer-running Nurses’ Health Study, overseen by Harvard-affiliated researchers. It found no increased risk of breast cancer in women who had taken estrogen for less than 10 years. But for women who had been on estrogen for at least 15 years, the risk of hormonally driven breast cancer, the most common type in the United States, climbed 48 percent. At the 20-year mark, the risk of any type of breast cancer rose 42 percent.