Zestril is an ACE inhibitor used to treat high blood pressure. It may also be used to treat congestive heart disease and other conditions as determined by your doctor. Note: Lisinopril 30mg is not available in Canada. To achieve 30mg, 1 1/2 tablets of the 20mg. Zestril is used in the treatment of high blood pressure. It is effective when used alone or when combined with other high blood pressure prescriptions. It may also be used with other prescriptions in the treatment of heart failure, and may be given within 24 hours of a heart attack to improve chances of survival. Zestril is a type of drug called an ACE inhibitor. It works by reducing production of a substance that increases salt and water retention in your body. Zestril is used to treat high blood pressure and heart failure.