Via FDA Press - Patients should avoid using the stomach acid reducer Prilosec/Prilosec OTC (omeprazole) with the anti-clotting drug Plavix (clopidogrel), the U.S. Food and Drug Administration warned on Nov. 17. New data suggest that when patients take both Prilosec and Plavix, Plavix's ability to block platelet aggregation (anti-clotting effect) may be reduced by about half.

"Both of these drugs, when used properly, provide significant benefits to patients" said Mary Ross Southworth, Pharm.D., of the Division of Cardiovascular and Renal Products in the FD's Center for Drug Evaluation and Research. However, patients at risk for heart attacks or strokes who use Plavix to prevent platelet aggregation will not get the full effect of this medicine if they are also taking Prilosec.

Plavix is used to prevent blood clots that could lead to heart attacks or strokes in at-risk patients. Omeprazole, the active ingredient of Prilosec and Prilosec OTC, is a proton pump inhibitor (PPI) used to reduce the production of stomach acid and prevent stomach irritation.

Plavix does not have anti-clotting effects until it is converted or metabolized into its active form with the help of the liver enzyme, CYP2C19. Prilosec blocks this enzyme, thereby reducing the effectiveness of Plavix.

After issuing an Early Communication in January about possible drug interactions involving Plavix, the FDA requested new studies from the drug's manufacturers, sanofi-aventis and Bristol-Myers-Squibb. These new studies support the existence of a significant interaction that could negatively impact a persons health. Based on the current scientific information, the Plavix label has been updated with new warnings about the use of Prilosec and other drugs that inhibit the CYP2C19 enzyme and that could interact with Plavix in the same way.

The new studies compared people who took Plavix and Prilosec together with people who took Plavix alone. A reduction in Plavix's anti-clotting effect was found in those individuals who took the combination. Similar results were seen irrespective of whether the two drugs were taken at the same or different times of day.

It is unknown how other PPIs may interfere with Plavix. Other drugs that should not be used with Plavix because they may have a similar interaction with CYP2C19 include Nexium (esomeprazole), Tagamet and Tagamet HB (cimetidine), Diflucan (fluconazole), Nizoral (ketoconazole), VFEND (voriconazole), Intelence (etravirine), Felbatol (felbamate), Prozac, Serafem, Symbyax (fluoxetine), Luvox (fluvoxamine) and Ticlid (ticlopidine).

Patients who take Plavix and need to take a drug to reduce stomach acid should discuss their therapy with a health care professional. Zantac (ranitidine), Pepcid (famotidine), Axid (nizatidine), and antacids do not inhibit the CYP2C19 enzyme and aren't expected to interfere with the anti-clotting activity of Plavix.

Plavix's manufacturers have agreed to continue conducting studies to explore this and other drug interactions. When the FDA has reviewed additional data, the agency will communicate any new recommendations or conclusions.