Terfenadine was an antihistamine formerly used for the treatment of allergic conditions. Terfenadine is a prodrug, generally completely metabolized to the active form fexofenadine in the liver by the enzyme cytochrome P450 CYP3A4 isoform. Due to its near complete metabolism by the liver immediately after leaving the gut, terfenadine normally is not measurable in the plasma. Terfenadine itself, however, is cardiotoxic at higher doses, while its major active metabolite is not. Terfenadine, in addition to its antihistamine effects, also acts as a potassium channel blocker. Since its active metabolite is not a potassium channel blocker, there is no cardiotoxicity associated with fexofenadine. Toxicity is possible after years of continued use with no previous problems as a result of an interaction with other medications such as erythromycin, or foods such as grapefruit.
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