Nitroglycerin is in a group of drugs called nitrates. Nitroglycerin dilates (widens) blood vessels, making it easier for blood to flow through them and easier for the heart to pump. Nitroglycerin transdermal (skin patch) is used to prevent attacks of chest pain (angina). This medicine will not treat an angina attack that has already begun. Nitroglycerin transdermal may also be used for purposes not listed in this medication guide.