Zestril is a medication used to treat high blood pressure and heart failure, and it is also given to improve survival after a heart attack. It contains the active ingredient Lisinopril, which is an angiotensin-converting enzyme (ACE) inhibitor. It works by relaxing blood vessels, allowing blood to flow more smoothly and the heart to pump more efficiently. The dosage is typically one tablet taken orally once a day, or as advised by a healthcare professional. Side effects can include dizziness, lightheadedness, or dry cough. It's important to take this medication consistently, even if you feel well, as high blood pressure often has no symptoms.