Saying the drug may be less effective in preventing heart attacks in people who cannot metabolize it properly, US health officials Friday ordered the makers of the Plavix anti-blood clotting agent to add a boxed warning to its packets.
The new boxed warning ordered by the Food and Drug Administration is a further step up from just mentioning the drawback on the label which has been required since May 2009.
"We want to highlight this warning to make sure health care professionals use the best information possible to treat their patients," said Mary Ross Southworth, analyst in the FDA's Center for Drug Evaluation and Research.
The FDA said an estimated two to 14 percent of the US population are poor metabolisers, who have reduced functioning of the CYP2C19 liver enzyme that converts Plavix to its active form.
"As a result, Plavix may be less effective in altering platelet activity in those people," the FDA said.
"These 'poor metabolizers' may not receive the full benefit of Plavix treatment and may remain at risk for heart attack, stroke, and cardiovascular death."
The FDA said consumers can be tested to determine if they are poor metabolizers.
Plavix, one of the world's top selling drugs, is marketed jointly by US-based Bristol-Myers Squibb and France's Sanofi. It reduces the risk of heart attack and stroke by keeping blood platelets from sticking together to cause clots.