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Wednesday, March 4, 1998 - Page updated at 12:00 AM

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Blood Thinner, Tylenol Called A Risky Mix

The AP

CHICAGO - People who often take Tylenol or any other form of acetaminophen along with a widely used blood-thinning drug may raise their risk of deadly internal bleeding, researchers say.

The study confirms a hazard that has been known for decades but has been little appreciated.

Frequent doses of acetaminophen, the active ingredient in Tylenol, increase the blood-thinning effect of warfarin, a drug used to prevent strokes and heart attacks, researchers warned in today's Journal of the American Medical Association.

"While acetaminophen generally is a very safe drug for pain and fever," people on warfarin who take acetaminophen for at least seven days in a row should be closely watched for bleeding, said the study's lead author, Dr. Elaine Hylek of Massachusetts General Hospital.

Warfarin, also known as Coumadin, is taken by millions of people with a heart condition called atrial fibrillation to prevent the formation of blood clots that could cause strokes. It is also taken to prevent clots around replacement heart valves and to dissolve clots in the legs or lungs.

People taking warfarin often choose acetaminophen for everyday pain relief in the mistaken belief that it will not thin their blood further, the researchers said. Aspirin is known to thin the blood, and both aspirin and ibuprofen, another widely used pain reliever, can damage the stomach lining.

Warfarin's effects can be altered by many things - illness, other medications, even certain foods - so treatment involves a delicate balance: Too little blood-thinning can cause a clot that could travel to the brain and cause a stroke; too much thinning can lead to deadly internal bleeding.

Patients on long-term warfarin treatment usually get a monthly blood test called an INR, or international normalized ratio, that measures blood thinness.

The researchers surveyed 289 patients within 24 hours after they had gotten INR tests at Massachusetts General Hospital during regular outpatient visits. Among the patients, 93 had INRs greater than 6, a level associated with increased bleeding risk. Of the remainder, 196 had INRs in an optimum range of 2 to 3.

Those who took the equivalent of at least four regular-strength (325 milligram) acetaminophen tablets per day for seven days or longer had a tenfold increase in the likelihood of an INR of 6 or more.

The risk dropped with lower dosages, and no increased risk of an elevated INR was seen in patients taking six or fewer tablets per week.

Copyright (c) 1998 Seattle Times Company, All Rights Reserved.

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