Co-Diovan 80/12.5mg Tablets contain two active substances called valsartan and hydrochlorothiazide. Both of these substances help to control high blood pressure (hypertension). Valsartan belongs to a class of medicines known as “angiotensin II receptor antagonists”, which help to control high blood pressure. Angiotensin II is a substance in the body that causes vessels to tighten, thus causing your blood pressure to increase. Valsartan works by blocking the effect of angiotensin II. As a result, blood vessels relax and blood pressure is lowered. Hydrochlorothiazide belongs to a group of medicines called thiazide diuretics. Hydrochlorothiazide increases urine output, which also lowers blood pressure.