Terfenadine (1 I), a selective HI-histamine receptor antagonist? is a widely used antihistamine3 which is devoid of CNS effe~ ta.~ The compound is almost completely biotransformed by hepatic cytochrome P4503 to the pharmacologically active acid metabolite lo3 and the inactive metabolite azacyclonol (a, a-diphenyl-4-piperidinemethanol) in the first pas~.~ 9~ Recently, there have been reports which suggest that terfenadine is ...