Angiotensin II receptor blockers (ARBs) are medications that lower blood pressure and are commonly prescribed for the treatment of high blood pressure (hypertension) and heart failure. These medications produce similar effects to ACE inhibitors but by a different mechanism.

Angiotensin II is a substance in the body that signals blood vessels to constrict or narrow. Both ACE inhibitors and ARBs inhibit the effect of angiotensin II and therefore cause blood vessels to widen, reducing blood pressure and the workload of the heart. However, whereas ACE inhibitors work by lowering levels of angiotensin II, ARBs prevent (or block) angiotensin II substance from entering receptors in the smooth muscles of the heart and  blood vessels.

In addition, ACE inhibitors decrease the breakdown of bradykinin, a protein that has a powerful influence on the contraction of smooth muscle, such as the heart. ARBs have no effect on bradykinin.

Despite their similarities, these two types of medications are not interchangeable. Research is still trying to determine exactly which patients are more likely to benefit from ARBs. ARBs are generally more expensive than ACE inhibitors, but do not cause the coughing that is a common side effect of ACE inhibitors. This may be related to their lack of effect on bradykinin. 

Research into the potential benefits of ARBs for high blood pressure, heart failure and other cardiac conditions is still ongoing. Trials are under way to examine whether ARBs can also be used in combination with other blood pressure lowering medications – a practice that is becoming more frequent with ACE inhibitors. It appears that ARBs are more effective in some cases when used in conjunction with a diuretic, another drug used for high blood pressure. Currently, ARBs are primarily used in patients who cannot tolerate the side effects produced by ACE inhibitors.