US regulators have notified healthcare professionals with new safety information concerning an interaction between Bristol-Myers Squibb and Sanofi-Aventis’ blockbuster bloodthinner Plavix and proton pump inhibitors, especially AstraZeneca’s Prilosec.

The US Food and Drug Administration says new data show that when Plavix (clopidogrel) and Prilosec (omeprazole) are taken together, the effectiveness of the former is reduced. The agency adds that patients at risk for heart attacks or strokes who use clopidogrel to prevent blood clots “will not get the full effect of this medicine” if they are also taking Prilosec, noting that separating the dose of the two treatments in time “will not reduce this drug interaction”.

The FDA added that other drugs that are “expected to have a similar effect and should be avoided in combination with clopidogrel” include cimetidine, fluconazole, ketoconazole, voriconazole, etravirine, felbamate, fluoxetine, fluvoxamine and ticlopidine.

Earlier this year, B-MS and Sanofi updated Plavix's label to advise against using it in combination with PPIs, and the firms say it will now be strengthened to indicate that these drugs should be avoided completely.