Plavix is prescribed to heart disease patients to prevent dangerous blood clots that can increase risk for heart attack, stroke and death. (Mark Lennihan/Associated Press)

The blood thinner Plavix may not be activated fully in some patients, which increases their risk for heart attack and stroke, the U.S. Food and Drug Administration warns.

The regulator added its strongest warning to the label of the drug on Friday.

Plavix, known generically as clopidogrel, is prescribed to heart disease patients to prevent dangerous blood clots that can increase risk for heart attack, stroke and death.

"Because the patient makes less of the active form there is less anti-platelet effect in the blood and the patient may not receive the full benefit of Plavix treatment," Mary Ross Southworth of the FDA's Center for Drug Evaluation and Research told reporters in a teleconference.

An estimated two to 14 per cent of the population are "poor metabolizers" of Plavix. In these people, the liver does not convert Plavix into its active form effectively, and they may remain at risk for blood clots.

The FDA recommended that health-care professionals consider alternative doses for such patients, or look at other anti-platelet medications.

"Patients should not stop taking Plavix unless told to do so by their health-care professional," the agency advised. "They should talk with their health-care professional if they have any concerns about Plavix, or to find out if they should be tested for being a poor metabolizer."

Genetic testing premature: doctor

Genetic tests are available to assess if a patient is a poor metabolizer at a cost of around $500 US. Laboratories vary in how long it takes to get results.

"I think based on this people will do more genetic testing, but I think it's premature to say that everyone who gets Plavix needs to be tested," said Dr. Louis Teichholz, head of cardiology at Hackensack University Medical Center.

A spokeswoman for New York-based Bristol-Myers said the company would add the new labelling to Plavix bottles over the next two months.

The product's label first included information about poor metabolizers in May 2009.

Since then, evidence has mounted on the association between poor metabolizers with the CYPC219 type of liver enzyme, taking Plavix and poor treatment outcomes. After reviewing all the data, the FDA felt a stronger warning was needed to alert health-care providers to consider alternative treatments, Southworth said.

Plavix had global sales of $8.6 billion US in 2008 — the world's second best-selling drug behind Pfizer's cholesterol lowering drug Lipitor.

With files from The Associated Press