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General wisdom has it that terbinafine, a commonly used oral treatment for dermatophyte infections, has no effect on P450 cytochromes. This study examined terbinafine's effect on CYP2D6, an important cytochrome P450 that metabolizes more than 35 drugs.
CYP2D6 activity was tested in nine subjects who received 250 mg of oral terbinafine for two weeks. By genotyping and phenotyping, six had fully active CYP2D6, representing the 93% of the population who are extensive metabolizers. The other three had virtually no CYP2D6 activity, representing the 7% who are poor metabolizers. Terbinafine had no effect on CYP2D6 activity in the poor metabolizers but strongly inhibited activity in the extensive metabolizers. Among the latter, CYP2D6 was 20 to 270…