These can be divided into the pharmacodynamic, pharmacokinetic and formulation interactions. Pharmacodynamic Beta-blockers will potentiate the SA and AV node effects of verapamil and diltiazem, which increases the risk of bradycardia, AV block, severe hypotension and heart failure. Therefore, these drugs must not be used concomitantly. Nifedipine is widely used with beta-blockers due to the clinical benefits. For many patients there are no adverse events, but it should be remembered that there have been reports of severe hypotension and impairment of left ventricular function.
Amiodarone increases the risk of bradycardia, AV block and heart muscle depression.
This effect mainly occurs with diltiazem and verapamil. Concomitant use of flecainide with verapamil increases the risk of myocardial depression and asystole. Digoxin may potentiate the effects of diltiazem and verapamil on the SA node and AV node. Pharmacokinetic Plasma concentrations of quinidine are reduced by nifedipine and increased by verapamil. Both diltiazem and nifedipine block liver metabolism of phenytoin resulting in raised plasma levels.