calcium-channel blocker, any of a class of drugs used in treating hypertension, angina pectoris, and certain arrhythmias. They prevent the calcium ions needed for muscle contraction from entering the cells of smooth and cardiac muscle. This causes blood vessel walls to relax and blood to flow more freely to the heart, lowering blood pressure and relieving anginal pain. Some calcium-channel blockers, such as Procardia (nifedipine), slow the electrical impulses that run through heart muscle, thus regulating arrhythmias. Cardizem (diltiazem) is a common calcium-channel blocker.