diltiazem and carvedilol

Additive negative chronotropic, dromotropic and inotropic effects on the heart. Diltiazem also inhibits CYP3A4, which may increase plasma concentrations of carvedilol (a CYP3A4 and CYP2D6 substrate), though the pharmacodynamic effect is the dominant clinical concern.

Combined use can cause bradycardia, first-degree or higher AV block, worsening heart failure and hypotension, particularly in patients with pre-existing conduction abnormalities, reduced left ventricular function or volume depletion. The risk is increased with higher doses and when other negative chronotropic agents are used.

Use with caution. Assess baseline heart rate, blood pressure, ECG and left ventricular function. Consider initiating at lower doses and titrating slowly. Monitor heart rate, blood pressure and for symptoms of heart failure or syncope, particularly during the first few days after starting or changing doses. If resting heart rate falls below 50 bpm or symptomatic bradycardia occurs, reduce the dose or discontinue one of the agents.

The interaction is well established in drug interaction databases and textbooks. Clinical reports and pharmacokinetic studies support an additive effect on heart rate and blood pressure. The combination is sometimes used deliberately in specific settings (e.g., hypertension and angina), but requires careful patient selection and monitoring. No distinction between formulations of diltiazem is clinically relevant for this interaction.