Combipres is used to treat high blood pressure (hypertension). Lowering high blood pressure helps prevent strokes, heart attacks, and kidney problems.This product contains 2 medications: clonidine and chlorthalidone. Clonidine belongs to a class of drugs (central alpha agonists) that act in the brain to lower blood pressure. It works by relaxing blood vessels so blood can flow more easily. Chlorthalidone is a "water pill" (diuretic) and causes your body to get rid of extra salt and water. It also helps to relax the blood vessels so that blood can flow more easily. These medications are used together when 1 drug alone is not controlling your blood pressure.