Angiotensin-converting enzyme inhibitors, also called ACE inhibitors, are drugs used to treat high blood pressure and heart failure. ACE inhibitors s alters the body's ability to produce angiotensin II, a hormone that causes the arteries to narrow. By blocking the making of angiotensin, these drugs help the blood vessels relax and widen, which lowers blood pressure, increases blood flow to the heart and reduces the heart's workload.