New data indicates that omeprazole (Prilosec/Prilosec OTC), a medicine used to reduce stomach acid, also reduces the anti-blood clotting effect of clopidogrel (Plavix), a medication taken by patients at risk for heart attacks or stroke, by almost half when the two drugs are taken by the same patient, according to a new Public Health Advisory issued by the Food and Drug Administration (FDA). This drug interaction occurs because omeprazole blocks the conversion of clopidogrel into its active form. The interaction is not reduced if the doses of clopidogrel and omeprazole are separated with time.

Since clopidogrel may cause bleeding in the stomach, omeprazole may be used in combination to reduce the production of stomach acid and prevent stomach bleeding. Other medications that are expected to have the same effect and should be avoided in combination with clopidogrel include cimetidine, fluconazole, ketoconazole, voriconazole, etravirine, felbamate, fluoxetine, fluvoxamine, and ticlopidine.

Patients using clopidogrel should consult with their health care provider if they are current taking or considering taking omeprazole, including Prilosec OTC, and always consult with their health care provider before starting or stopping any medication. This includes telling their doctors about any over-the-counter medications they are taking before starting or while using clopidogrel.

Patients who are taking clopidogrel and need a medication to reduce stomach acid can use antacids such as Maalox or Mylanta, or most acid reducers such as Zantac (ranitidine), Pepcid (famotidine), or Axiid (nizatidine) because the FDA does not believe that these medications will interfere with the anti-clotting activity of clopidogrel. However, Patients who use clopidogrel should avoid Tagamet and Tagamet HB (cimetidine). Many of the products listed above are available both over-the-counter and by prescription.