Nitroglycerin, also known as glyceryl trinitrate (GTN), is a medication used for heart failure, high blood pressure, anal fissures, painful periods, and to treat and prevent chest pain caused by decreased blood flow to the heart (angina) or due to the recreational use of cocaine. This includes chest pain from a heart attack. It is taken by mouth, under the tongue, applied to the skin, or by injection into a vein.Common side effects include headache and low blood pressure. The low blood pressure can be severe. It is unclear if use in pregnancy is safe for the baby. It should not be used together with medications within the sildenafil (PDE5 inhibitor) family due to the risk of low blood pressure. Nitroglycerin is in the nitrate family of medications. While it is not entirely clear how it works, it is believed to function by dilating blood vessels.Nitroglycerin was written about as early as 1846 and came into medical use in 1878. It is on the World Health Organizations List of Essential Medicines. The drug nitroglycerin (GTN) is a dilute form of the same chemical used as the explosive, nitroglycerin. Dilution makes it non-explosive. In 2019, it was the 184th most commonly prescribed medication in the United States, with more than 3 million prescriptions. Medical uses
Nitroglycerin is used for the treatment of angina, acute myocardial infarction, severe hypertension, and acute coronary artery spasms. It may be administered intravenously, as a sublingual spray, or as a patch applied to the skin.
Tolerance
After long-term use for chronic conditions, nitrate tolerance—tolerance to agents such as GTN— may develop in a patient, reducing its effectiveness. Tolerance is defined as the loss of symptomatic and hemodynamic effects of GTN and/or the need for higher doses of the drug to achieve the same effects, and was first described soon after the introduction of GTN in cardiovascular therapy. Studies have shown that nitrate tolerance is associated with vascular abnormalities which have the potential to worsen patients prognosis. These include endothelial and autonomic dysfunction.The mechanisms of nitrate tolerance have been investigated over the last 30 years, and several hypotheses to explain tolerance have been offered, including: 

plasma volume expansion
impaired transformation of GTN into NO or related species
counteraction of GTN vasodilation by neurohormonal activation
oxidative stressRecent evidence suggests that deleterious GTN-induced production of oxygen free radicals might induce a number of abnormalities, include those described above, so that the oxidative stress hypothesis might represent a unifying principle. Adverse events
Glyceryl trinitrate can cause severe hypotension, reflex tachycardia, and severe headaches that necessitate analgesic intervention for pain relief, the painful nature of which can have a marked negative effect on patient compliance. GTN also can cause severe hypotension, circulatory collapse, and death if used together with vasodilator drugs that are used for erectile dysfunction, such as sildenafil, tadalafil, and vardenafil.GTN transdermal patches should be removed before defibrillation due to the risk of explosion and/or burns, but investigations have concluded that GTN patch explosions during defibrillation were due to voltage breakdown involving the metal mesh in some patches.