Tiazac Extended Release Capsules Cardizem, Cartia XT, Dilacor XR, Dilt-CD, Diltia XT, Diltzac, Matzim LA, Taztia XT, Tiazac Tiazac (diltiazem) belongs to a group of drugs called calcium channel blockers. It works by relaxing the muscles of your heart and blood vessels. Cardizem (diltiazem) is a calcium channel blocker. It works by relaxing the muscles of your heart and blood vessels. Tiazac is used to treat hypertension (high blood pressure) Tiazac is also used for the treatment of chronic stable angina. (chest pain).