SUNDAY, Jan. 11 (HealthDayNews) -- Warfarin is an anticoagulant -- or "blood thinner" -- that's frequently prescribed for people with atrial fibrillation.

An estimated 2.2 million Americans have the cardiac condition, which causes the heart's two upper chambers to flutter, rather than beat efficiently. This leaves people vulnerable to a blood clot in the brain that can result in an ischemic stroke, the most common type of stroke.

That's where warfarin comes in -- helping to prevent those clots.

But, ironically, as use of warfarin has risen in recent years, so has the number of another type of stroke called hemorrhagic stroke, which occurs when a blood vessel ruptures in the brain.

That doesn't mean that warfarin should be abandoned -- far from it, experts say.

"Decisions about warfarin use should be made by patients after discussion with their doctor," said Dr. Matthew Flaherty, a University of Cincinnati neurologist who led a study published last year in the journal Neurology about the rising rate of brain hemorrhages linked to anticoagulants.

The study looked at medical data from people in the Cincinnati area who had been hospitalized with a first-time brain hemorrhage in the years 1988, 1993-94 and 1999. It found that the annual rate of hemorrhagic stroke linked to warfarin use was 0.8 cases per 100,000 in 1988. But, that number had risen to 4.4 cases per 100,000 by 1999.

When Flaherty and his colleagues dug deeper, however, they found that the rise in the rate of hemorrhagic strokes in people 80 years of age and older was even greater -- from 2.5 per 100,000 in 1988 to 45.9 per 100,000 in 1999.

"Warfarin is very effective for preventing ischemic strokes in this setting but carries a risk of bleeding. For most patients, the benefit of warfarin more than offsets the risk," Flaherty said.