The risk of breast cancer in women using continuous combined hormone therapy begins to return to normal after the therapy is stopped, according to researchers at the National Institute of Child Health and Human Development (NICHD).

A daily, combined dose of estrogen and progestin increases the risk of breast cancer in postmenopausal women but the risk returns to normal about six months after women stop taking the hormones, according to the report published in Obstetrics and Gynecology.

"It is reassuring that breast cancer risk begins to return to normal six months after women stop combined dose estrogen-progestin therapy," said Dr. Duane Alexander, Director of the NICHD. "Women, in consultation with their physicians, need to make the most informed decision possible. The study authors have provided them with one more piece of important information."

"In planning the NICHD study, we sought to learn as much as we could about the risks associated with the various kinds of hormone therapy," said Robert Spirtas, DrPH, Chief of NICHD's Contraception and Reproductive Health Branch and senior author of the study. "At the time, little information existed on whether combined hormone therapy posed the same risks as estrogen therapy alone."

Researchers questioned 1,870 women who had been diagnosed with breast cancer about their use of hormones and other potential risk factors for breast cancer. The women were then compared to a similar group of 1,953 women who had not developed breast cancer.

"Our data suggest a positive association between continuous combined hormone replacement therapy and breast cancer risk among current, longer term users," wrote the authors. "Progestin administered in an uninterrupted regimen may be a contributing factor."

Investigators found that women on continuous combined therapy for five years or more were 1.54 times more likely to develop breast cancer than women not taking the therapy. The risk of breast cancer increased the longer the women used this form of therapy. However, six months after the women stopped taking the hormones, their risk of breast cancer began to return to normal. This was true for women who took the hormone therapy for five years or longer before stopping, as well as for those who took the therapy for only about six months.

"The NICHD study also included former users, and our data indicate that the increased risk associated with use of continuous combined hormone therapy began returning to normal about six months after the women stopped using them," said Spirtas.

The study also found that women who took estrogen-progestin therapy for only five to 14 days per month were not at an increased risk of breast cancer. Also, the study did not find any additional risk of breast cancer in women who took estrogen alone.

Other sources: National Institutes of Health