A new European study on the effects of hormone replacement therapy indicates that progestin, not estrogen, may cause breast cancer in women. Presented Tuesday at the annual meeting of the Swedish Society of Medicine in Goteborg, the study involved 30,000 women, including 3,500 taking hormone-replacement medications to relieve symptoms of menopause. Out of 395 women who took progestin for more than four years, 25 developed breast cancer, while only nine of 322 women taking estrogen alone contracted the disorder.

Through analysis of the data, researchers determined that women who took progestin for four or more years had triple the normal risk for breast cancer. Critics of the study say the number of women examined was too small for physicians to make a definitive link between progestin therapy and breast cancer. Sales of hormone-replacement medications plummeted in recent months after a report linked the drugs to heart disease signs, symptoms of blood clots and breast cancer.