A study of nearly 20,000 women suggests that the risk of breast cancer can be reduced by a drug that carries fewer side-effects than tamoxifen, the current medical standard.

Raloxifene is a close relative of tamoxifen, and is just as effective at cutting the risk of invasive breast cancer in half among post-menopausal women who take it every day, the National Cancer Institute said Monday in releasing the results.

However, unlike tamoxifen, it is less likely to prompt uterine cancer or dangerous blood clots.

"Women who were prospectively and randomly assigned to take raloxifene daily, and who were followed for an average of about four years, had 36 per cent fewer uterine cancers and 29 per cent fewer blood clots than the women who were assigned to take tamoxifen," said a news release.

The $88-million US study involved 19,747 women aged 35 and up.

Cancer prevention chief calls study 'good news'

Those figures could lead to raloxifene replacing tamoxifen when it comes to protecting post-menopausal women who have either already developed tumours or are at a high risk of doing so because of their family and medical histories.

"For many women, raloxifene's benefits will outweigh its risks in a way that tamoxifen's benefits do not," Dr. Leslie Ford, the cancer prevention chief at the Washington, D.C.-based institute, said in a statement

"It's good news, because we're giving you a choice with fewer side-effects," she added later in an interview with the Associated Press.

Drug already sold to treat osteoporosis

Raloxifene is used as a treatment for the bone-thinning effects of osteoporosis, sold under the brand name Evista.

Its manufacturer, Eli Lilly & Co., must apply to the Food and Drug Administration for the drug to be approved in the United States for marketing as a preventive treatment for breast cancer.

Health Canada must give similar approval in this country.

In the meantime, the National Cancer Institute says women already taking tamoxifen without experiencing side-effects have no reason to switch.

Medical experts say older women are more likely to develop breast cancer if they:

  • Have a family history of the disease.
  • Experienced their first menstrual periods by age 12.
  • Have either never had a child or were over 30 when they gave birth to their first child.
  • Have a history of drinking two or more alcoholic beverages each day.

Each year, about 20,000 Canadian women are diagnosed with the disease and about 5,000 die.