The analysis was part of the National Institute of Child Health and Human Development (NICHD) Women’s Contraceptive and Reproductive Experiences Study.

“It is reassuring that breast cancer risk begins to return to normal six months after women stop combined dose estrogen-progestin therapy,” said Director of the NICHD. “Women, in consultation with their physicians, need to make the most informed decision possible.”

This was the first large clinical trial to assess the risks and benefits of continuous combined hormone therapy. In this form of therapy, women take a combination of the hormones estrogen and progestin. Essentially, the hormone estrogen relieves such symptoms of menopause as hot flashes, night sweats, sleeplessness, and vaginal dryness. When taken alone estrogen also increases a woman’s risk for cancer of the uterine lining, or endometrium. Combining estrogen with progestin virtually eliminates the risk of endometrial cancer.

During the study, women used continuous combined hormone therapy and researchers monitored their health. When it became clear that the women were developing breast cancer at higher than normal rates, the researchers ceased the study immediately. Because of this, researchers were unable to tell yet whether the women in the study face any increased risk of breast cancer now that they stopped taking the hormones.

In contrast, researchers for the NICHD study began by questioning women who had been diagnosed with breast cancer about their hormone use and other potential risk factors for breast cancer. These women were then compared to a similar group of women who had not developed breast cancer. The data suggests a positive association between continuous combined HRT and breast cancer risk among current, longer term users and that Progestin administered in an uninterrupted regimen may be a contributing factor.

A total of 3823 postmenopausal white and black women were analyzed for the study. In all, the medical histories of 1870 women who had developed breast cancer were compared to the histories of 1953 women who did not have breast cancer.

The researchers found that women on continuous combined therapy for 5 years or more were 1.5 times more likely to developing breast cancer than other women their age not on this form of therapy. The risk of breast cancer increased the longer the women used this form of therapy. However, six months after the women discontinued the combined therapy; their risk of breast cancer began to return to normal. This held true for women who took the hormones for 5 years or longer before stopping, as well as for women who took the hormones for only about 6 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 6 months after the women stopped using them.

Estrogen alone may be prescribed to women whose uterus has been surgically removed, as these women no longer have a risk of endometrial cancer. The NICHD study did not find any additional risk of breast cancer in women who took estrogen alone. A recent study by the NCI found, however, that women taking estrogen alone may be at increased risk for cancer of the ovary.