The new label warns that normal doses of Plavix have a potentially deadly lack of effect in 2% to 14% of patients.
Such patients are so-called "poor metabolizers" who carry a variant CYP2C19 gene affecting the enzyme that converts Plavix into its active form. The frequency is about 2% of Caucasians, 4% of blacks, and 14% of Chinese.
However, a 2010 study published in the NEJM contradicted the statement above:
It has been suggested that clopidogrel may be less effective in reducing the rate of cardiovascular events among persons who are carriers of loss-of-function CYP2C19 alleles that are associated with reduced conversion of clopidogrel to its active metabolite.
Among patients with acute coronary syndromes or atrial fibrillation, the effect of clopidogrel as compared with placebo is consistent, irrespective of CYP2C19 loss-of-function carrier status.
New Plavix Warning: Lack of Effect in Many People. WebMD.
Effects of CYP2C19 Genotype on Outcomes of Clopidogrel Treatment. NEJM, 2010.
Image source: A box of Plavix. Wikipedia, Trounce, Creative Commons Attribution-Share Alike 2.5 Generic license.