THURSDAY, July 28 (HealthDay News) -- Women who took the cancer-suppressing drug tamoxifen for five years after a breast cancer diagnosis were nearly 40 percent less likely to have the cancer return, and that protection lasted for more than a decade after they stopped taking the drug, a new study finds.

Researchers analyzed the results of about 20 randomized controlled trials on a five-year course of tamoxifen vs. not taking the drug. The trials involved 21,000 women from a dozen countries around the world, including the United States, Europe, China and Japan.

Some 15 years after their diagnosis -- and 10 years after they stopped taking the drug -- women who took tamoxifen still had one-third lower risk of dying than women who didn't take it.

"It's a remarkable drug," said study author Dr. Christina Davies, a lead investigator with the Early Breast Cancer Trialists Collaborative Group, which was established some 25 years ago to conduct periodic reviews of research on breast cancer from around the world. "It has probably saved more lives than any other oncological drug ever."

Of 10,645 women who took tamoxifen, about 26 percent had a relapse at the 10-year-mark, compared to 40 percent who didn't take the medication. By 15 years, 33 percent of women who took the drug had their cancer return, compared to 46 percent who didn't.

The statistics were similar when it came to death rates. After a decade, about 25 percent of women who didn't take the drug had died compared to 18 percent of those who did take it; at 15 years, 33 percent who didn't take the drug died compared to 24 percent of those who took tamoxifen.

"They not only gained the benefits while they were taking the drug, but for many years afterward," Davies said.

The study is published in the July 28 online issue of The Lancet.

Tamoxifen has been widely used for more than 30 years to treat the most common type of breast cancer, estrogen-receptor positive tumors.

The drug works by inhibiting the activity of estrogen, a female hormone that can drive the growth of breast cancer tumors. The drug is most often prescribed as a once-a-day pill for younger women with breast cancer.

Older, postmenopausal women are now often prescribed a newer class of drugs called aromatase inhibitors, which block estrogen released in body fat, experts said. Aromatase inhibitors are most easily used in women who no longer have ovaries that are producing estrogen, Davies said.

One reason for the switch to aromatase inhibitors: Prior research, as well as the current study, found that tamoxifen raises the risk of cancer of the lining of the uterus (endometrial cancer) and life-threatening blood clots in the lungs. The analysis found the added risk from tamoxifen to older women was small, and in younger women it was "almost non-existent," Davies said.

"The benefits greatly outweigh the risks," Davies said.

Dr. Len Lichtenfeld, deputy chief medical officer of the American Cancer Society, said studies such as this that look at death rates over the long-term are valuable.

"The study shows that tamoxifen as an adjuvant [additional] therapy for breast cancer has had a very successful track record that has been sustained over 30 years of use in the clinic, even though it's used less now," he said.

The analysis found tamoxifen worked equally well in women who underwent chemotherapy and radiation in addition to surgery, Davies noted. Another benefit: tamoxifen is inexpensive. Davies estimated a five-year course of the drug costs about $150, of particular importance in developing nations where breast cancer rates have risen dramatically, she said.

Many of the women in the analysis, she noted, failed to take the full five-year course of the drug, so it's possible the protective effect from fully taking the medication as prescribed might be even greater. The findings raise the question of whether, say, a 10-year course of tamoxifen might be even more beneficial than five years.

About half of the women worldwide diagnosed with breast cancer are pre-menopausal, she said. "For those women, tamoxifen is the main treatment option," she said.

About two-thirds of breast cancers in the United States are estrogen-positive. The drug worked even on those whose tumors are "weakly positive," she noted.

More information

The U.S. National Cancer Institute has more on tamoxifen.