Isoptin SR Drug Interactions

The simultaneous administration of Isoptin SR and cardioactive drugs (eg, β-receptor blockers, antiarrhythmics) or inhalation anaesthetics may lead to a mutual enhancement of the cardiovascular effects (AV blockade, bradycardia, hypotension, heart failure). Until further data are obtained, combined therapy of verapamil and quinidine in patients with hypertrophic obstructive cardiomyopathy should probably be avoided.
IV β-receptor blockers should not be given to patients under treatment with Isoptin SR.
Isoptin SR may intensify the blood pressure-lowering effect of other antihypertensives.
Rises in digoxin plasma levels under concomitant administration of verapamil have been reported. Physicians should be on the alert for symptoms of possible digoxin toxicity. The digitalis level should be determined and the glycoside dose reduced, if required.
There have also been occasional reports on interactions with carbamazepine (potentiated by verapamil), lithium (attenuated by verapamil) and rifampicin (effects of verapamil attenuated).
Phenobarbital therapy may increase verapamil clearance. Verapamil therapy may increase serum levels of cyclosporin. Verapamil may inhibit the clearance and increase the plasma levels of theophylline.