Through a new boxed warning, the FDA is alerting healthcare professionals about a subgroup of patients who cannot effectively metabolize the anti-platelet drug Plavix (clopidogrel).

These patients, called “poor metabolizers,” have little or no activity of the liver enzyme CYP2C19, which converts Plavix to its active form, so they may not experience the full anti-clotting benefits of the drug.

Practitioners should know that tests are available to identify genetic differences in CYP2C19 function and thus identify poor metabolizers. They should consider using other anti-platelet medication or an alternative dosing strategy for these patients. And although raising the dose of Plavix in poor metabolizers can increase anti-platelet response, an appropriate dose regimen has not been established in a clinical trial.