GTN (glyceryl trinitrate) in tablet or spray form is used for the immediate relief of chest pain due to angina. The tablets have a short shelf-life and should be thrown away eight weeks after opening the bottle. The spray lasts longer, but you need to check the expiry date on the bottle. Angina is a pain caused by the heart not having enough oxygen. GTN works by opening up the blood vessels taking oxygen to the heart. This allows the heart to get more oxygen and should result in the angina pain going away.