An ACE inhibitor (angiotensin-converting enzyme) is a type of medicine used to treat heart attacks, heart failure, or a decreased function of the left side of your heart. ACE inhibitors can help reduce the risk of death from a heart attack if taken within 24 hours of the first symptoms of a heart attack. Continued use may help prevent heart failure. ACE inhibitors work by stopping the production of a hormone that narrows blood vessels. This helps reduce the pressure in your heart and lower blood pressure. If you have a heart attack, you should get a prescription for ACE inhibitors before you leave the hospital.