ibuprofen can cripple aspirin's ability 
to protect against a heart attack

The popular pain killer ibuprofen can cripple aspirin's ability to protect against a heart attack, researchers report in the New England Journal of Medicine. The results, if confirmed, would have major implications for millions of people who take a small dose of aspirin every day to thin the blood, but who also take ibuprofen, to cope with conditions such as arthritis.
(BBC DEC 19, 2001)