For about a decade, doctors have recognized that certain medications can actually lower a woman's risk of developing breast cancer. These drugs all work by blocking the effects of the female hormone estrogen, which appears to stimulate the growth of breast cancer cells.

Tamoxifen

The drug tamoxifen was first used to treat women with breast cancer. Researchers found that tamoxifen could prevent breast cancer from coming back after surgery was performed. However, the researchers also noted that tamoxifen reduced the chance that a second cancer would develop in the unaffected breast.

Further studies have shown that women at high risk who take tamoxifen for five years can reduce the risk of breast cancer by as much as 40 percent. Unfortunately, tamoxifen also has serious side effects, including an increased risk of life-threatening blood clots and cancer of the uterus. For this reason, most doctors recommend that only women who are at high risk of breast cancer take tamoxifen for its preventive benefits. Furthermore, the serious side effects of tamoxifen appear more common in older women than in younger women.

Tamoxifen is currently the only drug that is approved by the U.S. Food and Drug Administration for the primary prevention of breast cancer in high-risk women. However, several other drugs have shown some promise:

Raloxifene (Evista)

Raloxifene (Evista) is chemically related to tamoxifen. Both are in a class of drugs known as selective estrogen receptor modulators, or SERMs. Raloxifene was originally developed for the prevention and treatment of osteoporosis, but, like tamoxifen, it lowers the risk of breast cancer. Raloxifene also increases the risk of blood clots but not as much as tamoxifen. In addition, the risk of cancer of the uterus is much lower with raloxifene compared to tamoxifen.

Anastrozole (Arimidex) And Related Drugs

Anastrozole (Arimidex) and the related drugs exemestane (Aromasin) and letrozole (Femara) are in a class of drugs known as aromatase inhibitors. They were originally developed to treat breast cancer that had spread through the body, but preliminary studies suggest that they also can lower a woman's risk of developing breast cancer. Aromatase inhibitors may cause osteoporosis as well as bothersome side effects such as hot flashes, but they do not appear to trigger blood clots or uterine cancer. Studies are under way to see if these drugs are more or less effective than tamoxifen at preventing breast cancer in women at high risk.

Women who are concerned about their risk of developing breast cancer — particularly if they have a strong family history of breast cancer or have had abnormal cells seen on previous breast biopsies — should speak with their doctors about taking one of these preventive drugs. However, because these drugs have serious side effects, it is important to weigh the risks and benefits before starting treatment.