Using morphine along with clopidogrel may adversely affect the action of clopidogrel, potentially leading to treatment failure, according to a study published in the Feb. 25 issue of the Journal of the American College of Cardiology.

Researchers from the Medical University of Vienna in Austria hypothesized that the action of morphine may lower plasma levels of the active metabolite of clopidogrel (Plavix, Bristol-Myers Squibb/Sanofi-Aventis) and its antiplatelet effect and sought to evaluate possible drug interactions.

The investigators, led by Eva-Louise Hobl, PhD, gave 24 healthy participants a loading dose of clopidogrel along with either a placebo or a 5 mg dose of intravenous morphine. They assessed pharmacokinetics by using liquid chromatography tandem mass spectrometry and the effects of clopidogrel using platelet function tests.

Morphine slowed down clopidogrel absorption and reduced the concentration of its active metabolite. It also delayed the antiplatelet effects by an average of two hours. Residual platelet aggregation was higher one to four hours after morphine administration.

The authors argued that their findings suggest the two drugs should not be administered together. “More potent P2Y12 inhibitors might provide greater efficacy when morphine is injected, but their interaction with morphine should be evaluated in further trials,” they wrote.