Aspirin and a common blood-thinning drug may be able to prevent strokes in as many as 75,000 Americans a year, studies show.

The blood-thinner cut the risk of stroke by 67 percent, said Dr. Roger E. Kelley, a neurologist at the University of Miami."The data also suggest that aspirin is effective and is a reasonable alternative," Kelley said Thursday at the annual stroke meeting of the American Heart Association. Aspirin, which also has blood-thinning or anti-clotting properties, cut the risk by 42 percent, he said.

In separate studies, researchers in Boston and Canada found similar results with the blood-thinning or anti-clotting drug warfarin, long used for such conditions as phlebitis. At much higher strengths it is used as a rat poison. They did not study aspirin.

The Boston study and the study Kelley was part of showed such dramatic results that they were abruptly halted because doctors decided it would be unethical to continue denying the drugs to subjects in the control group, Kelley said.

The Canadian study was then stopped in the wake of that decision, he said.

Doctors treating heart attack victims have been widely prescribing aspirin to reduce the risk of additional attacks, and a study published in July said warfarin has a similar effect.

Dr. Melvin Cheitlin, a cardiologist at the University of California at San Francisco, praised the stroke studies but said more research is needed to determine two things: the appropriate doses of warfarin or aspirin, and the length of time they should be given.