Study: Acetaminophen, warfarin a dangerous combination

• High-risk recipe: 4 tablets a day, 7 days a week
• Occasional doses OK

Researchers say that those who take Tylenol or other forms of acetaminophen along with the widely used blood-thinning drug warfarin may face the risk of serious internal bleeding.

The study confirms a hazard that has been known for three decades but has been not been communicated to patients and seems to have been forgotten by many doctors.

Taking frequent doses of acetaminophen, the active ingredient in Tylenol, increases the blood-thinning effect of warfarin, according to an article published in Journal of the American Medical Association.

Millions of Americans take warfarin -- which is also known as Coumadin -- for heart conditions 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.

"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'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.

High-risk recipe: 4 tablets a day, 7 days a week

Aspirin is known to thin the blood, but both aspirin and ibuprofen, another widely used pain reliever, can damage the stomach lining.

Hylek says that some of those using warfarin take acetaminophen for pain relief in the mistaken belief that it will not thin their blood further.

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

In their study, the researchers surveyed 289 patients within 24 hours after they had INR tests and found that 93 had INRs greater than 6, a level associated with increased bleeding risk. The remaining 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 there was no increased risk for patients taking six or fewer tablets per week.

Occasional doses OK

Doctors say that for those taking warfarin who have a fever or headache and cannot take aspirin or ibuprofen, occasional doses of acetaminophen are recommended.

Dr. William R. Bell of Johns Hopkins University School of Medicine said if patients must take warfarin and acetaminophen simultaneously, their blood thinness should be measured once or twice a week and their INRs should not exceed 4.