ACE Inhibitors (Angiotensin-Converting Enzyme) Inhibitors are drugs used to treat high blood pressure and heart failure. They stop the body’s ability to produce angiotensin II, a natural substance that causes blood vessels to tighten (contract). ACE inhibitors relax and expand (dilate) blood vessels, allowing blood to flow more easily. This increases the supply of blood and oxygen to the heart, making the heart work more easily and efficiently. (Also known as antihypertensive drugs.)