tacrolimus and verapamil

Verapamil inhibits cytochrome P450 3A4 (CYP3A4) and P-glycoprotein (P-gp), both of which are involved in the metabolism and efflux of tacrolimus. This inhibition decreases the clearance of tacrolimus, leading to increased systemic exposure.

Increased tacrolimus concentrations predispose the patient to dose-related toxicities, including nephrotoxicity, neurotoxicity, hyperglycaemia, and hypertension. The risk is particularly important given tacrolimus has a narrow therapeutic index. Even modest rises in trough levels can precipitate toxicity.

Avoid co-prescription if possible. If combination is considered necessary, measure tacrolimus trough levels before and after starting verapamil, and at intervals thereafter. Reduce the tacrolimus dose as needed to maintain therapeutic trough concentrations. Monitor renal function and watch for signs of tacrolimus toxicity.

The interaction is well documented in multiple pharmacokinetic studies and case reports, showing significant increases in tacrolimus concentrations when verapamil is introduced.