Experts are hailing the results of an international study that shows a previously shelved drug works significantly better than the most commonly used treatment for women with advanced breast cancer. Researchers report the drug reduces the risk of breast cancer coming back.

Women diagnosed with advanced breast cancer that is likely to spread after surgery are treated with a drug known as tamoxifen for five years. But after awhile, the drug can stop working, and the cancer can return.

Researchers report this week in the New England Journal of Medicine that another drug given to women after treatment with tamoxifen reduces by one-third the likelihood that the cancer will come back.

The drug, called exemestane, was developed in the 1970's, but was never used because of toxic side effects. Newer versions of the drug do not cause the same risks.

Carina Biggs is chief of breast surgery at Maimonides Medical Center in New York. Dr. Biggs says both exemestane and tamoxifen are effective against breast cancers caused by the female hormone estrogen, which is responsible for an estimated 70 percent of all breast cancers.

Dr. Biggs says both drugs block estrogen production, but in different ways. Tamoxifen inhibits secretion of the hormone by cells; exemestane thwarts its production by the adrenal glands.

This one-two punch is now used to treat a number of diseases, including AIDS. Dr. Biggs says it also appears to work in the treatment of breast cancer.

"The hypothesis has been that tumors may become resistant to the mechanism of one drug," explains Dr. Biggs. "And by using a combination of two, or even three medications, the tumor can be eradicated by two or three mechanisms."

In the study, 5,000 women in 37 countries were recruited to either receive tamoxifen for five years, or tamoxifen for two-to-three years followed by exemestane.

After five years, just over 90 percent of women who received exemestane after being treated with tamoxifen were cancer-free, compared with 87 percent who only received tamoxifen. Of these, 67 women in the tamoxifen group died of breast cancer compared with 54 women who received both drugs.

Cheryl Perkins of the Susan G. Komen Foundation, which raises money for breast cancer research, says survivors are constantly worried that their disease will come back. "If this evidence continues to hold up, this will be a wonderful option for women in that situation," he said.

Experts say tamoxifen remains the gold standard of treatment for advanced breast cancer, although doctors may begin adding exemestane, promising new hope for women with breast cancer.