The FDA has added a boxed warning to the label for clopidogrel (Plavix, Bristol-Myers Squibb/Sanofi Aventis), the most widely sold anti-clotting medication, for patients who do not effectively metabolize the drug (i.e., poor metabolizers) and therefore may not receive the full benefits of the drug.

According to the agency, for Plavix to work, enzymes in the liver (particularly CYP2C19) must metabolize the drug to its active form.

Patients who are poor metabolizers of the drug do not effectively convert Plavix to its active form, the FDA said. In these patients, Plavix has less effect on platelets, and therefore less ability to prevent heart attack, stroke and cardiovascular death.

The FDA said it is estimated that between 2 to 14 percent of the population are poor metabolizers; the rate varies based on racial background.

The boxed warning in the drug label will include information to:

• Warn about reduced effectiveness in patients who are poor metabolizers of Plavix. Poor metabolizers do not effectively convert Plavix to its active form in the body.
• Inform healthcare professionals that tests are available to identify genetic differences in CYP2C19 function.
• Advise healthcare professionals to consider use of other anti-platelet medications or alternative dosing strategies for Plavix in patients identified as poor metabolizers.