The warning alerts patients and healthcare professionals that 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.
However, Plavix does not have its anti-platelet effects until it is metabolized into its active form by the liver enzyme, CYP2C19, the FDA said in a statement.
"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," the FDA statement 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."
It is estimated that 2 percent to 14 percent of the U.S. population are poor metabolizers. The FDA recommends that healthcare professionals consider alternative dosing of Plavix for these patients, or consider using other anti-platelet medications.
Tests are available to assess CYP2C19 genotype to determine if a patient is a poor metabolizer, the FDA said.