propranolol and diltiazem

Both drugs have additive negative chronotropic, dromotropic and inotropic effects, leading to depression of the sinoatrial and atrioventricular nodes and reduced myocardial contractility. In addition, diltiazem inhibits CYP3A4, which reduces the metabolism of propranolol and increases its plasma concentration.

Combined use increases the risk of severe bradycardia, atrioventricular block, hypotension and exacerbation of heart failure. The effect is more pronounced in patients with pre-existing conduction abnormalities, reduced left ventricular function or those taking other cardiodepressant agents.

Avoid co-prescription where possible, particularly in patients with heart failure, conduction disease or hypotension. If unavoidable, initiate at low doses, monitor heart rate, blood pressure and ECG regularly, and adjust therapy based on findings. Consider using a dihydropyridine calcium channel blocker instead of diltiazem if a calcium antagonist is required.

This interaction is well documented in standard pharmacology texts and drug interaction databases. Clinical reports and case series describe clinically significant bradycardia and heart block with this combination.