OLMESARTAN CH 40 Tablet is a combination of two medicines namely Olmesartan and chlorthalidone. It is generally used when one medicine alone is not controlling your blood pressure effectively. Olmesartan works by relaxing and widening your blood vessels so your blood can flow more easily. This will lower your blood pressure. Chlorthalidone 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.