NEW YORK (Bloomberg) — Plavix, the blood thinner made by Sanofi-Aventis SA and Bristol-Myers Squibb Co., will carry new US warnings about the risk of interaction with an over-the- counter heartburn medication.

AstraZeneca Plc's Prilosec reduces the anti-blood clotting benefits of Plavix by almost half when the drugs are taken together, according to a notice posted yesterday on the Food and Drug Administration's website. The agency ordered Sanofi, of Paris, and New York-based Bristol-Myers to update their prescribing information and study the potential for other drug interactions.

Plavix, known generically as clopidogrel, is prescribed to millions of Americans after surgery to prevent heart attacks and strokes. Global sales rose 17 percent last year to $9.5 billion, second only to Pfizer Inc.'s cholesterol pill Lipitor. Drugs such as Prilosec, or omeprazole, are sometimes prescribed to Plavix patients to ease the digestive side effects. Regulators began probing the safety of the combination in January.

"Patients using clopidogrel should consult with their health-care provider if they are currently taking or considering taking omeprazole, including Prilosec OTC," the FDA said in its notice. "Both clopidogrel and omeprazole can provide significant benefits to patients, and patients should always consult with their health-care professional before starting or stopping any medication."