Combining aspirin (search) with the painkiller Bextra (search) may magnify the heart-related risks associated with Bextra and similar drugs.
They suggest that combining Bextra with aspirin may increase the risk of blood clots that could trigger a heart attack or stroke.
Last month, the FDA added anew warning to Bextra, which states that Bextra should not be used in patients undergoing heart bypass surgery after a clinical trial showed an increase in heart problems in bypass patients using Bextra.
A similar drug, Vioxx (search), was removed from the market in September 2004 because of an increase in heart attacks and stroke in patients taking the drug for at least 18 months.
A third, similar drug, Celebrex, was linked to heart attacks last month as well.
Like the other Cox-2 inhibitors, Vioxx and Celebrex (search), it was designed to treat arthritis and other painful conditions.
Older drugs like ibuprofen (search) and naproxen (search) treat pain and inflammation by blocking two enzymes, Cox-1 and Cox-2.
In the studies, which appear in the Jan. 17 issue of Circulation: Journal of the American Heart Association, researchers examined some potential mechanisms behind the increase in heart problems associated with Cox-2 inhibitors.
In the first study, researchers studied the effects of Bextra and aspirin on hardening of the arteries (atherosclerosis) in mice prone to the condition.
They found that low-dose aspirin slowed the development of atherosclerosis in the mice, but it seemed ineffective once the disease was established.
"We were amazed," says researcher Karine Egan, PhD, of the University of Pennsylvania, in a news release.
By adding a Cox-2 inhibitor, researchers say the beneficial effects of aspirin may be lost by tipping the balance in favor of the Cox-1 enzyme.