Cyclosporin-phenytoin interaction: re-evaluation using metabolite data.

1 Freeman et al. (1984) evoked enzyme induction to explain the lower plasma concentration of cyclosporin following phenytoin co-administration in man. 2 We have examined the whole blood concentration data of two metabolites of cyclosporin, as well as unchanged drug, all measured by h.p.l.c., associated with the above mentioned study. 3 Phenytoin produced no significant effect on either the terminal half-life of both metabolites or the ratio of area under the concentration-time curve of metabolite to parent drug. 4 These data strongly suggest that phenytoin reduces the absorption of cyclosporin; they do not generally support the idea that phenytoin induces cyclosporin metabolism.