WASHINGTON (AP) _ The federal government has notified thousands of women of a breakthrough in the fight against cancer, saying a study for the first time shows that a drug may prevent breast cancer.

The National Cancer Institute, the agency that coordinates the nation's cancer programs, said its six-year study showed that taking the drug tamoxifen cut cancer rates by nearly half among women who were considered at risk of getting the disease.

Critics, however, raised concerns about the drug's links to cancer in the uterine lining and to blood clots.

The institute has mailed letters announcing the breakthrough to the 13,000 women in the United States and Canada who participated in the study, which was first reported Sunday by The Philadelphia Inquirer.

``This is now the first study in the world to show that a drug can reduce the incidence of breast cancer,'' the letter stated.

Details of the study were to be made public today.

Sona Thakkar, a spokeswoman for the National Cancer Institute, said researchers were not discussing the report until it was formally released this afternoon. The report is expected to address the concerns about uterine cancer and the formation of blood clots.

One of the study's participants, 45-year-old Patricia Lorah of Reading, Pa., lost her mother and grandmother to breast cancer. The deaths prompted her to take part in one of the largest cancer prevention trials ever.

``I'm just thrilled. Wow!'' Lorah said. ``This is almost overwhelming.''

Tamoxifen, made by Wilmington, Del.-based Zeneca Pharmaceuticals, is widely used to prevent the spread or return of breast cancer.

Women at risk of getting the disease because of family history, precancerous breast lesions or age were randomly assigned to five years on either a placebo pill or tamoxifen.

According to the institute, the drug reduced the rate of expected breast cancers from one in 130 women to one in 236 during the study _ a decline of about 45 percent. Tamoxifen slips into estrogen receptors of breast cancer cells and locks up the cells, preventing them from growing and dividing.

Tamoxifen has been associated with increased risks for cancer in the uterine lining and with blood clots in the lungs. Those risks prompted The National Women's Health Network in Washington to criticize the study.

``If this turns out to be a good risk-benefit ratio for some women, that will be good news,'' said Cindy Pearson, the group's executive director.

But researchers must tell what they know about ``the cost of this benefit,'' she said. ``Did any women die of anything caused by tamoxifen?''

Leslie Ford, the National Cancer Institute official overseeing the trial, predicted four years ago that if 16,000 women were recruited, tamoxifen would be expected to prevent 120 to 125 breast cancer cases, while producing 58 to 80 new cases of uterine cancer _ which is far easier to detect early and cure than breast cancer.

In 1994, the study was suspended during congressional hearings into four uterine cancer deaths in another study of breast cancer treatment using tamoxifen. And University of Pittsburgh surgeon Bernard Fisher, coordinator of the study, was investigated for reports that he was slow to address research problems.

That did not prevent women from staying in the study.

``I never considered dropping out. My thoughts were more like, `Maybe I'll save my child's life and my own and other people's,''' said Fern Maklin, 49, of Newtown, Pa.

Study participants will be tracked for at least two more years.

Researchers still are analyzing the data. Medical recommendations for using tamoxifen to prevent cancer are still being developed, according to the letter.

The women now can go to the 270 medical centers participating in the study to find out whether they were taking tamoxifen or the placebo.

``I'm hoping I was on tamoxifen, but if I was in the placebo arm, I am going to ask my physician to put me on tamoxifen because I really believe in it,'' said Helen Wilson, 48, of North Wales, Pa.