Jan. 26 (Bloomberg) -- Bristol-Myers Squibb Co. and Sanofi- Aventis SA’s Plavix is being examined by U.S. regulators to determine whether the blood-thinner becomes less effective in patients using heartburn drugs or with genetic variations.

Research published last year suggests that so-called proton pump inhibitors such as AstraZeneca Plc’s Nexium or Eisai Co.’s Aciphex and genetic differences alter how the body breaks down Plavix, according to a notice posted today on the Food and Drug Administration’s Web site. Bristol-Myers and Sanofi have agreed to conduct new studies to better understand how the medicines and heredity affect how well Plavix works, the agency said.

Plavix was the second-biggest medicine in 2007, and adding restrictions to its prescribing information may erode sales of a critical component to New York-based Bristol-Myers and Paris- based Sanofi’s future earnings growth. The FDA said it may take “several months” before a conclusion is reached.

“The FDA recognizes the importance of obtaining these data promptly,” the agency said in its notice. “The FDA will review the new information expeditiously upon receipt from the drug manufacturers and will communicate its conclusions and any recommendations to the public at that time.”

Meanwhile, doctors should continue to prescribe Plavix and re-evaluate whether their patients need proton pump inhibitors, the agency said. Patients should talk to their doctor if they’re already taking or considering a heartburn drug, including over- the-counter Prilosec, sold by Procter & Gamble Co., the FDA said.

Shares Fall

Bristol-Myers fell 14 cents to $22.25 at 4:01 p.m. in New York Stock Exchange composite trading after rising as much as 2.1 percent before the FDA announcement. Sanofi fell 90 cents, or 1.9 percent, to 46.55 euros at the close of trading in Paris.

More than 90 million patients around the world have taken Plavix since 1998 to reduce the risk of blood clots that can cause heart attacks or strokes. Global sales rose 28 percent to $8.08 billion in 2007, according to data compiled by Bloomberg.

The companies “are working with the FDA and conducting studies to obtain additional information that will allow us to understand and characterize the factors that may influence this complex issue,” said Laura Hortas, a spokeswoman for Bristol- Myers, in a phone interview today. She wouldn’t say what specific studies have been requested as a matter of company policy.

The FDA began issuing these types of “Early Communications about an Ongoing Safety Review” in 2007 after lawmakers and consumer groups said the agency failed to quickly identify and disclose side effects linked to popular drugs including Merck & Co.’s painkiller Vioxx and GlaxoSmithKline Plc’s diabetes pill Avandia.

To contact the reporter on this story: Catherine Larkin in Washington at clarkin4@bloomberg.net.

Last Updated: January 26, 2009 16:47 EST