Irbesartan is an angiotensin receptor blocker (ARB). It relaxes the blood vessel by blocking the action of a chemical that usually makes blood vessels tighter. This lowers the blood pressure, allowing the blood to flow more smoothly to different organs and the heart to pump more efficiently. Hydrochlorothiazide is known as a diuretic (water pill) which increases urine output thereby lowering blood pressure. If your blood pressure is controlled you are less at risk of having a heart attack, stroke, or kidney problems. The medicine must be taken regularly as prescribed to be effective. You do not usually feel any direct benefit from taking this medicine, but it works in the long term to keep you well.