A boxed warning was added on Friday to the anti-blood clotting drug Plavix, or clopidogrel, or clopidogrel by U. S. Food and Drug Administration officials.

The drug, which reduces the risk of heart attack, unstable angina, stroke and cardiovascular death in patients with cardiovascular disease by making platelets less likely to form blood clots, can be less effective in people who cannot metabolize the drug to convert it to its active form, says the warning.

The FDA said in a statement that Plavix does not have its anti-platelet effects until it is metabolized into its active form by the liver enzyme, CYP2C19.

The FDA statement further said," People who have reduced functioning of their CYP2C19 liver enzyme cannot effectively convert Plavix to its active form. As a result, Plavix may be less effective in altering platelet activity in those people. These 'poor metabolizers' may not receive the full benefit of Plavix treatment and may remain at risk for heart attack, stroke and cardiovascular death."

According to an estimate2 percent to 14 percent of the U. S. population is poor metabolizers. The FDA recommends that healthcare professionals consider alternative dosing of Plavix for these patients, or consider using other anti-platelet medications.

The FDA also said that tests are available to assess CYP2C19 genotype to determine if a patient is a poor metabolizer. (With Inputs from Agencies)