This case shows the importance of examining a metabolic drug interaction inan appropriate population. Asians have up to 20% incidence of poor metabolism (presence of two null alleles for CYP2C19) and a substantial incidence ofpenetrance of one null allele for CYP2C19, making them intermediatemetabolizers of CYP2C19 substrates. The use of this Asian population underestimated the extent of this drug interaction.