The pain reliever Ibuprofin may block the heart-protecting effects of ASA pills in people who take both popular pain relievers.

ASA in Aspirin acts as a blood thinner to help prevent clots that can cause heart attacks. Ibuprofin, found in Motrin and Advil, is a nonsteroidal anti-inflammatory drug widely used for arthritis and other aches and pains.

Doctors and patients need to know about potential drug interactions. Bayer, the makers of Aspirin, helped fund a study to find out what happens when Aspirin and Ibuprofen are taken at the same time.

A new factor to consider when taking painkillers

"We found a common dose of Ibuprofen interfered with the ability of Aspirin to afford cardio protection," said University of Pennsylvania pharmacologist Dr. Garrett Fitzgerald.

The study found both painkillers attack two different versions of the same blood clotting enzyme, called cyclooxygenase-1.

When the two drugs are taken together, Ibuprofin blocks the channel so Aspirin can't do its job of protecting against blood clots.

Dr Gordon Crelinsten

"This isn't an indictment of all nonsteroidals, but it does give one pause," wrote Dr. Leslie Crofford, an arthritis specialist at the University of Michigan in an editorial accompanying the study.

Crofford said researchers should study humans to verify the whether the test tube results apply in us.

Montreal cardiologist Dr. Gordon Crelinsten said until the human studies are done, he'll take the test tube results into account when he prescribes Aspirin to heart patients.

"I would inform them of the potential interaction between Ibuprofen and Aspirin and suggest they try another drug that the interaction has not been demonstrated in," said Crelinsten.

The study found acetaminophen, found in Tylenol, did not not work against Aspirin.

The drug interaction study appears in Thursday's issue of the New England Journal of Medicine .