A new drug cuts by nearly half the risk that older women who have survived early-stage breast cancer will suffer a relapse, according to the results of a large international study released Thursday.

The study, which was halted early so that the results could be made public, has the potential to change the way thousands of women are treated for one of the most common and feared malignancies, researchers said.

“The results of this study unquestionably offer new hope to hundreds of thousands of breast cancer patients and their families,” said Paul Goss of the Princess Margaret Hospital in Toronto, who led the study.

The drug, letrozole, for the first time offers many post-menopausal women who have undergone the standard treatment for early-stage breast cancer a way to reduce their risk of a recurrence for more than five years.

About 211,300 American women are expected to be diagnosed with breast cancer this year, and 40,200 are expected to die, making the disease the second most common cancer and cause of cancer deaths among women. The new treatment could benefit nearly 100,000 of those women, experts estimated.

Currently, women diagnosed with early breast cancer undergo surgery to remove the tumor, sometimes receive chemotherapy to kill cancer cells that may have spread, and then typically take the drug tamoxifen for five years to minimize the chances that their cancer will return. But taking tamoxifen for more than five years provides no added benefit and may even be dangerous.

Because many women who survive early-stage breast cancer suffer recurrences after five or more years of survival, doctors have been urgently searching for ways they can continue to reduce the risk.

“This becomes a dark cloud that hangs over many patients,” Goss said at a news conference in Toronto, where the findings were announced. “I don’t think any of us understand what it’s like to wake up every day waiting for the other shoe to drop.”

Estrogen fuels the growth of breast cancer cells, increasing the chances of a recurrence. Tamoxifen is a weaker version of estrogen that blocks the action of the hormone in the body. Letrozole, which is sold as Femara, is one of a new generation of breast-cancer preventives known as aromatase inhibitors. It blocks the creation of estrogen in the body in the first place.

Letrozole has already been approved for the treatment of advanced breast cancer. Because studies have shown that combining tamoxifen and letrozole appears to protect women from recurrences for longer periods than using tamoxifen alone, researchers decided to see if the drug would lengthen the protection if administered for five years after women have completed five years on tamoxifen.

In the new study, 5,187 women in the United States, Canada and Europe — most of whom had tumors stimulated by estrogen — took letrozole or a placebo once a day after undergoing the standard treatment, including five years on tamoxifen.

The women were supposed to have been followed for five years. But after they had been taking the letrozole for an average of about 2 1/2 years, a panel of experts monitoring the study decided that the benefits were so striking that it would be unethical to continue giving half the participants a useless substitute.

The National Cancer Institutes in the United States and Canada and the Canadian Cancer Society, which coordinated and funded the research with support from Novartis Pharmaceuticals, the drug’s manufacturer, agreed to stop the trial and release the results. The details will be published in the Nov. 6 issue of the New England Journal of Medicine, which posted the study and two accompanying editorials Thursday on its Web site.

When the study was terminated, 207 new cancers had been diagnosed — 132 among those receiving the placebo, but just 75 among the women taking letrozole — a 43 percent difference.

The risk of death from breast cancer was also nearly halved, but there were too few deaths for that finding to reach statistical significance, researchers said. The benefits, however, appeared to be equal for women whose cancer had spread beyond the breast and those whose cancer had not spread.

The drug reduced recurrences in the breast that was initially affected, in the other breast and in other parts of the body.

“This very important advance in breast cancer treatment will improve the outlook for many thousands of women,” said Andrew von Eschenbach, director of the U.S. National Cancer Institute.

The women taking letrozole were more likely to develop the bone-thinning disease osteoporosis, and to suffer broken bones. Researchers are studying ways to ameliorate that side effect. Women on the drug also had more hot flashes, as well as muscle and joint aches.