It is also used off-label as a preventive medication for cardizem mechanism of action. It is a Class 3 antianginal drug, and a Class IV antiarrhythmic.

Diltiazem is metabolized by, and acts as an inhibitor of the CYP3A4 enzyme, which can cause it to interact with a variety of other medications. Diltiazem is a potent vasodilator, increasing blood flow and variably decreasing the heart rate via strong depression of A-V node conduction. Diltiazem has negative inotropic, chronotropic, and dromotropic effects. A reflex sympathetic response, caused by the peripheral dilation of vessels and the resulting drop in blood pressure, works to counteract the negative inotropic, chronotropic and dromotropic effects of diltiazem.

Atrial fibrillation or atrial flutter is another indication. The initial bolus should be 0. Because of its vasodilatory effects, diltiazem is useful for treating hypertension. Calcium channel blockers are well tolerated, and especially effective in treating low-renin hypertension. In congestive heart failure, patients with reduced ventricular function may not be able to counteract the inotropic and chronotropic effects of diltiazem, the result being an even higher compromise of function.

With SA node or AV conduction disturbances, the use of diltiazem should be avoided in patients with SA or AV nodal abnormalities, because of its negative chronotropic and dromotropic effects. Low blood pressure patients, with systolic blood pressures below 90 mm Hg, should not be treated with diltiazem. Diltiazem may paradoxically increase ventricular rate in patients with Wolff-Parkinson-White syndrome because of accessory conduction pathways. Because of its inhibition of hepatic cytochromes CYP3A4, CYP2C9 and CYP2D6, there are a significant number of drug interactions. Intravenous diltiazem should be used with caution with beta-blockers because, while the combination is most potent at reducing heart rate, there are rare instances of dysrhythmia and AV node block.