A major clinical trial that began in 1992 provided the first evidence that women can do something to reduce their risk for breast cancer. The study, which involved more than 13,000 women, was called the Breast Cancer Prevention Trial, sponsored by the National Surgical Adjuvant Breast and Bowel Project (NSABP).

In 1998, the investigators found that the oral drug tamoxifen (tradename Nolvadex) actually reduced the chances of developing breast cancer by 49 percent in women considered to be at risk because of age or other risk factors, such as strong family history, previous breast cancer, or signs of precancerous conditions.

It had an even stronger 69 percent preventive effect on tumors that were identified as "estrogen-receptor-positive," or estrogen-sensitive tumors, since tamoxifen is an "anti-estrogen" drug. In hormone-sensitive cancer, cancerous cells are sensitive to female hormones and can be stimulated by estrogen.

After an average treatment time of over four years, there were 175 cases of breast cancer in the women who received the placebo, compared with 89 in the tamoxifen-treated group. The annual rate for developing breast cancer was reduced from 6.8 in 1000 women to 3.4. The risk was reduced for all age groups, but women over 60 derived a larger 55 percent reduction.

As a result of these findings, many physicians recommend that women of high risk take tamoxifen to help prevent breast cancer. However, the drug does have side effects, which some women find objectionable. The most serious possibility is the development of endometrial cancer, which increased more than twofold in the NSABP trial (from 0.9 to 2.3 cases per 1000 women). However, women were instructed to watch for signs of endometrial cancer, and all 36 cases that occurred during the study were found in early, curable stages.

Tamoxifen-treated women also developed more blood clots. The incidence of blood clots among women who use tamoxifen is similar to that of women who take hormone replacement therapy. A more common occurrence, however, was hot flashes, reported by almost half the women receiving tamoxifen. It is not recommended that women take estrogen replacement therapy to relieve hot flashes while on tamoxifen, since the aim of tamoxifen is to block estrogen.

Since the development of tamoxifen, a similar agent called raloxifene (Evista) has been proposed as potentially preventative as well. These two drugs are now being compared in the STAR (Study of Tamoxifen and Raloxifene) trial of 22,000 at-risk women.

Elizabeth Shaughnessy, M.D., assistant professor of surgery at The University Hospital in Cincinnati, is the local principal investigator for the STAR trial and the North American Fareston vs. Tamoxifen Adjuvant Trial (NAFTA). "If a woman is interested in taking tamoxifen or raloxifene to help prevent breast cancer, it is very important that she talk with her physician about the pros and cons of taking the drugs," says Dr. Shaughnessy. "Also, women should remember that the best way of fighting breast cancer is through early detection. That’s why it is so important that women ages 20 and older perform regular self breast examinations, and women ages 40 and over should receive regular mammograms."

More information on tamoxifen and raloxifene is available at the NSABP web site.