Telmisartan is a class of drugs called angiotensin receptor blockers (ARBs), which includes losartan, valsartan, irbesartan, and candesartan. ARBs block the ability of the chemical angiotensin II to constrict or squeeze arteries and veins. As a result, the arteries and veins enlarge and blood pressure falls. The reduced pressure in the arteries also makes it easier for the heart to pump blood.

Angiotensin II normally acts on special receptors in the body, with two main results. Firstly, it causes the peripheral blood vessels to narrow, and secondly, it stimulates the production of another hormone called aldosterone which causes salt and water to be retained by the kidneys, which increases the volume of fluid in the blood vessels. This drug blocks the receptors that angiotensin II acts on, and so prevents its actions. The main result of this is that the peripheral blood vessels are allowed to widen, which means that there is more space and less resistance in these blood vessels. This is the main mechanism by which the pressure in the blood vessels is lowered. Blocking the actions of angiotensin II also reduces the action of aldosterone on the kidneys. The result of this is an increase in the amount of fluid removed from the blood by the kidneys. This decreases the amount of fluid in the blood vessels, which also lessens the resistance and pressure in the blood vessels.