Diltiazem enhances the effects of triazolam by inhibiting its metabolism*

Abstract

Triazolam is metabolized by CYP3A4. Diltiazem is an inhibitor of this isozyme and interacts with midazolam, another substrate of this enzyme. Therefore the possible interaction between triazolam and diltiazem is worth investigation. A balanced, randomized, double-blind crossover study design was used, with an interval of 2 weeks between phases. Ten healthy volunteers were given 60 mg diltiazem orally or placebo three times a day for 2 days. On the second day they received a single 0.25 mg oral dose of triazolam, after which plasma samples were collected and effects of triazolam measured for up to 17 hours. Diltiazem increased the mean area under the triazolam concentration-time curve threefold (p < 0.001) and the elimination half-life (p < 0.001) and the peak plasma concentration of triazolam twofold (p < 0.005). The increased concentrations of triazolam during the diltiazem phase were associated with increased and prolonged pharmacodynamic effects. Diltiazem has a clinically significant interaction with oral triazolam. The data is highly suggestive that diltiazem inhibits the metabolism of triazolam during the first-pass and elimination phases. Prescription of triazolam should be avoided if a patient is using diltiazem or other potent inhibitors of CYP3A.