terbinafine and flecainide: label evidence and limits

The terbinafine tablet label names flecainide among drugs metabolized by CYP2D6. It advises careful monitoring with coadministration because terbinafine inhibits this enzyme.

Which products this evidence covers

Oral terbinafine tablets, not topical antifungal creams.

What this finding does and does not establish

The label discusses possible clinician-directed dose reduction of the CYP2D6 substrate. This comparison does not validate an individualized dose or CDI severity category.

Supporting label passage

In vivo studies have shown that terbinafine is an inhibitor of the CYP450 2D6 isozyme. Drugs predominantly metabolized by the CYP450 2D6 isozyme include the following drug classes: tricyclic antidepressants, selective serotonin reuptake inhibitors, beta-blockers, antiarrhythmics class 1C (e.g., flecainide and propafenone) and monoamine oxidase inhibitors Type B. Coadministration of terbinafine tablets should be done with careful monitoring and may require a reduction in dose of the 2D6-metabolized drug. In a study to assess the effects of terbinafine on desipramine in healthy volunteers characterized as normal metabolizers, the administration of terbinafine resulted in a 2-fold increase in C max and a 5-fold increase in area under the curve (AUC). In this study, these effects were shown to persist at the last observation at 4 weeks after discontinuation of terbinafine tablets. In studies in healthy subjects characterized as extensive metabolizers of dextromethorphan (antitussive drug and CYP2D6 probe substrate), terbinafine increases the dextromethorphan /dextrorphan metabolite ratio in urine by 16- to 97-fold on average. Thus, terbinafine may convert extensive CYP2D6 metabolizers to poor metabolizer status.

Read the product label on DailyMed. Section: 7.1 Drug-Drug Interactions. Compared label version 5, dated 2026-09-10.

How this brief was prepared

This brief uses an AI-assisted source comparison dated 2026-10-16. The label version and supporting passage were checked again on 2026-10-29. Independent clinical review has not been recorded. The comparison does not validate CDI's severity category or determine individual treatment.

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