ow doses of a cheap blood-thinning drug can safely prevent dangerous blood clots in the veins of people who are prone to them, doctors reported yesterday, saying the findings should prompt an immediate change in treatment for high-risk patients.

The drug, warfarin, also sold as Coumadin, has been in use since the 1950's and is available in generic form.

Until now, there has been no proven long-term therapy that is safe for high-risk patients. When a clot develops, people are usually given warfarin for three to six months, but then the drug is stopped, because it increases the risk of severe bleeding. Without warfarin, however, almost a third of the patients will form another clot within eight years.

The new study has found that after a few months of standard treatment with full doses of warfarin, clots can be prevented with lower doses, about half of what is normally used to treat someone with a clot. The lower doses provide a safe middle ground, reducing the risk of more clots without increasing the risk of hemorrhage.

The study supporting the long-term use of small doses of warfarin is to be published April 10 in The New England Journal of Medicine but was released yesterday and posted on the journal's Web site (www.nejm.com ) because the findings have immediate implications for patients.

The study itself was halted ahead of schedule by its sponsor, the National Heart, Lung and Blood Institute, because a safety board found such a benefit to treatment that it would have been unethical to keep giving placebos to people in the control group.

"This really ought to change the standard of care overnight," said Dr. Paul Ridker, the first author of the study and director of the center for cardiovascular disease prevention at Brigham and Women's Hospital in Boston.

"For the first time we have a study demonstrating a safe and effective treatment to prevent the recurrence of this condition," said Dr. Yves Rosenberg, a medical officer at the national institute. He said doctors should offer the treatment to all patients at risk for clots.

Blood clots, known as deep vein thrombosis, are considered a major public health problem in the United States. Estimates of the incidence vary, doctors say, because blood clots often go undiagnosed, including many fatal ones.

Dr. Rosenberg, a medical officer at the institute, said 250,000 to 500,000 Americans a year suffered from clots. Others suggest there are two million cases a year in the nation.

Half occur in surgical patients or people with cancer, other illnesses or trauma, and half occur for no known reason.

People at highest risk for developing blood clots are those who have had them in the past. The clots form most commonly in the legs, but about half break loose and reach the lungs, a condition called pulmonary embolism, which can be fatal. Blood clots in the lungs kill at least 10,000 Americans a year, and perhaps as many as 40,000, Dr. Rosenberg said.

Some people can develop clots from sitting or lying still for too long, even from sitting in cramped airplane seats on long flights — hence the name "economy class syndrome."

In recent years, the tendency to develop clots has been recognized as a chronic disease, sometimes due to hereditary blood disorders.

The study included 508 patients at 52 hospitals in the United States, Canada and Switzerland who were at high risk because they had already had blood clots, for no known reason. They were followed for an average of 2.1 years, some as long as 4.3 years. Half got low doses of warfarin, and half got placebos. Of the 253 on placebos, 37 developed blood clots, as compared with only 14 of 255 on the drug. The risk reduction was 64 percent.

The treatment was safe, the researchers said. Although more patients on warfarin had hemorrhages (five versus only two in the placebo group), the difference was not statistically significant. One patient had a stroke caused by a brain hemorrhage, but that person was in the placebo group, not on warfarin.

Dr. Ridker said he and his colleagues began the study in hope of finding a way to help patients who were living with the constant threat of forming dangerous blood clots.

"It's frightening," Dr. Ridker said. "We take care of a lot of these patients, they have these risk factors and don't know what to do. Patients are frustrated."

Dr. Ridker said he asked the National Institutes of Health to pay for the study because he assumed that drug companies, which underwrite many studies, would not be interested, since warfarin is generic and companies would not make money promoting it.

Dr. Ridker said: "I said to N.I.H., `Look, what do we do with these patients? Maybe a low dose will make just enough change in the coagulation system to prevent these blood clots from coming back, and maybe it will be safe."'

In an editorial accompanying the study in the journal, Dr. Andrew I. Schafer of the University of Pennsylvania said the study findings meant it would be reasonable for doctors to give low-dose warfarin to patients at risk for clots. But he also noted that the advice might change in the future, as more studies were done and as new drugs became available.