Older women looking for a medication to help lower their risk of invasive breast cancer now have another option available to them.

The US Food and Drug Administration (FDA) has approved raloxifene (Evista) for breast cancer risk reduction in two groups of postmenopausal women: those with the bone-thinning condition osteoporosis and those at high risk for invasive breast cancer. It joins tamoxifen as just the second so-called chemoprevention drug to be approved for breast cancer.

Under the brand name Evista, raloxifene is already approved, and widely prescribed, for the prevention and treatment of osteoporosis in postmenopausal women. Researchers had previously observed that these women -- roughly half a million at present -- had lower rates of breast cancer than the general population.

Raloxifene is a type of drug known as a selective estrogen receptor modulator, or SERM. In reducing the risk of invasive breast cancer, SERMs may act by blocking estrogen receptors in the breast.

The expanded use of raloxifene -- based on positive clinical studies of approximately 37,000 postmenopausal women over a 10-year period -- "provides an important new option for women at heightened risk of breast cancer," said Dr. Steven Galson, director of the FDA's Center for Drug Evaluation and Research.

In its announcement, the FDA noted that raloxifene can cause serious side effects, including blood clots in the legs and death due to stroke. Other potential side effects include hot flashes, leg cramps, swelling of the legs and feet, flu-like symptoms, joint pain, and sweating.

The FDA said women with current or prior blood clots in the legs, lungs, or eyes should not take raloxifene. It should not be taken with cholestyramine, a drug used to lower cholesterol levels, or with estrogens.

It also should not be taken by women who are or may become pregnant. At present, tamoxifen remains the drug of choice for women who have not gone through menopause and have a high breast cancer risk. Raloxifene's breast cancer risk reduction effects have not been studied in younger women.

According to American Cancer Society estimates, more than 178,000 women are expected to be diagnosed with invasive breast cancer this year, accounting for more than one-fourth of all cancers in women.