Researchers at the University of Michigan in Ann Arbor say their study used biochemical measurements and X-ray crystallography to discover Celebrex binds to COX-1 -- an enzyme that promotes clotting -- and slows aspirin's COX-1-blocking action.
"There are many people who take low-dose aspirin, perhaps as many as half of men over the age of 50. If they are also prescribed Celebrex for arthritis or other pain, our results suggest that the Celebrex will probably interfere with the aspirin's action," senior author William Smith said in a statement.
"The greatest risk is having people take Celebrex who are taking aspirin for cardiovascular problems that are known to be mitigated by aspirin, including patients with unstable angina or those at risk for a second heart attack."
Smith also finds more clumping of platelets -- the initial stage of clotting -- in blood from animals given Celebrex and low-dose aspirin than in animals given only low-dose aspirin.
Should this effect hold true in humans, it will be important to determine if a balance in dose and/or dose regimens could allow both the aspirin and Celebrex to be effective, Smith says.
The study is published in the Proceedings of the National Academy of Sciences.