Omeprazole, a proton pump inhibitor, may hamper the effectiveness of the anticlotting medication clopidogrel, and physicians should avoid using this combination of drugs, according to the US Food and Drug Administration (FDA).
Omeprazole inhibits a drug-metabolizing enzyme, CYP2C19, which converts clopidogrel into its active metabolite. Recent studies measuring the blood levels of clopidogrel's active ingredient in patients who took clopidogrel alone or combined with omeprazole found that individuals who took both drugs had 45% lower levels of clopidogrel's metabolite. Additionally, the effect on platelets was reduced by as much as 47% in patients who also took omeprazole. These reductions were seen both when the drugs were taken simultaneously and when they were taken with a 12-hour interval in between.
Kuehn BM. Clopidogrel Interaction. JAMA. 2010;303(4):322. doi:10.1001/jama.2009.1982