Adverse interaction between clonidine and verapamil

OBJECTIVE: To report two cases of a possible adverse interaction between clonidine and verapamil resulting in atrioventricular (AV) block in both patients and severe hypotension in one patient. CASE SUMMARIES: A 54-year-old woman with hyperaldosteronism was treated with verapamil 480 mg/d and spironolactone 100 mg/d. After the addition of a minimal dose of clonidine (0.15 mg bid), she developed complete AV block and severe hypotension, which resolved upon cessation of all medications. A 65-year-old woman was treated with extended-release verapamil 240 mg/d. After the addition of clonidine 0.15 mg bid she developed complete AV block, which resolved after all therapy was stopped. DISCUSSION: An adverse interaction between clonidine and verapamil has not been reported previously. We describe two such cases and discuss the various mechanisms that might cause such an interaction. Clinicians should be acquainted with this possibly fatal interaction between two commonly used antihypertensive drugs. CONCLUSIONS: Caution is recommended in combining clonidine and verapamil therapy, even in patients who do not have sinus or AV node dysfunction. The two drugs may act synergistically on both the AV node and the peripheral circulation.