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Nitrate vasodilator

Nitroglycerin

Nitroglycerin is an organic nitrate vasodilator used primarily for acute angina relief and heart failure management. It works by releasing nitric oxide, which dilates veins more than arteries, reducing preload. The most testable fact: nitroglycerin causes reflex tachycardia and must never be combined with phosphodiesterase-5 inhibitors due to risk of severe hypotension.

Practice questionsHigh-yield facts
Drug classOrganic nitrate vasodilator
MechanismReleases nitric oxide; activates guanylate cyclase; dilates veins
Main usesAcute angina, unstable angina, acute heart failure
Key adverse effectHeadache, hypotension, reflex tachycardia, nitrate tolerance
6 min readFor medical studentsUpdated 2026-10-14

Mechanism of action

Nitroglycerin is a prodrug that is converted to nitric oxide (NO) either spontaneously or via enzymatic pathways involving mitochondrial aldehyde dehydrogenase-2 (ALDH-2). Nitric oxide then activates guanylate cyclase in vascular smooth muscle, increasing cyclic GMP levels, which leads to dephosphorylation of myosin light chains and smooth muscle relaxation.

Nitroglycerin preferentially dilates venous capacitance vessels over arteries at therapeutic doses. This venodilation reduces venous return (preload), decreasing left ventricular end-diastolic pressure and myocardial oxygen demand. At higher doses, arterial dilation occurs, reducing afterload. Nitroglycerin also dilates coronary arteries, improving blood flow to ischemic myocardium, particularly by relieving coronary vasospasm.

The net hemodynamic effect is:

  • Decreased preload (primary effect)
  • Decreased afterload (at higher doses)
  • Reduced myocardial wall tension and oxygen consumption
  • Improved coronary perfusion

This combination makes nitroglycerin highly effective for relieving angina pectoris by rebalancing myocardial oxygen supply and demand.

High-yield for exams
  • Nitroglycerin releases nitric oxide, activating guanylate cyclase and increasing cGMP, causing venodilation and reduced preload.
  • Contraindicated with PDE-5 inhibitors (sildenafil, tadalafil) due to risk of severe, life-threatening hypotension.
  • Contraindicated in right ventricular infarction; RV function is preload-dependent, and reducing preload causes hemodynamic collapse.
  • Reflex tachycardia is a common side effect due to baroreceptor activation from hypotension.
  • Tolerance develops with continuous use; requires a 10-14 hour nitrate-free interval daily to restore efficacy.
  • Sublingual nitroglycerin relieves angina in 1-3 minutes; if pain persists after initial dose, suspect MI and seek emergency care.
  • Most common adverse effect is headache (up to 50%) from cerebral vasodilation.
  • Methemoglobinemia is a rare complication of high-dose IV nitroglycerin; presents with cyanosis unresponsive to oxygen; treat with methylene blue.

Indications

Nitroglycerin is indicated for:

  • Acute angina pectoris – sublingual or spray for immediate relief
  • Prophylaxis of angina – transdermal patches or oral sustained-release formulations
  • Unstable angina and acute coronary syndrome – intravenous administration
  • Acute decompensated heart failure – IV nitroglycerin reduces preload and improves pulmonary congestion
  • Hypertensive emergencies (off-label) – particularly when associated with myocardial ischemia
  • Esophageal spasm (off-label)
  • Anal fissures – topical nitroglycerin ointment promotes healing by increasing local blood flow

For acute angina, sublingual nitroglycerin typically relieves symptoms rapidly. If pain is unimproved after the initial administration, emergency care should be sought immediately as this may indicate myocardial infarction.

Chronic prophylactic use requires a nitrate-free interval (usually 10-14 hours overnight) to prevent tolerance.

Pharmacokinetics

Nitroglycerin exhibits route-dependent pharmacokinetics:

Absorption and bioavailability

  • Sublingual/buccal: Rapid absorption through oral mucosa, bypassing first-pass metabolism; onset within 1-3 minutes, duration 30-60 minutes
  • Oral: Extensive first-pass hepatic metabolism; bioavailability <10%; not used for acute therapy
  • Transdermal: Steady absorption through skin; onset 30-60 minutes, duration 8-12 hours (patch-dependent)
  • Intravenous: Immediate onset, duration 3-5 minutes after infusion stops; allows precise titration
  • Topical (ointment): Applied to skin or rectal area; variable absorption

Distribution and metabolism

Nitroglycerin is lipophilic and widely distributed. It undergoes rapid hepatic metabolism via glutathione-organic nitrate reductase and cytochrome P450 enzymes, producing inactive metabolites (1,2- and 1,3-dinitroglycerin) and active metabolites that contribute minimally to effect.

Elimination

Metabolites are renally excreted. Plasma half-life of nitroglycerin is approximately 1-4 minutes, but hemodynamic effects persist longer due to tissue distribution and active metabolites.

Tolerance develops with continuous exposure; a daily nitrate-free interval of 10-14 hours restores sensitivity.

Adverse effects

The most common adverse effects of nitroglycerin result from its vasodilatory action:

Cardiovascular

  • Hypotension – can be severe, especially in volume-depleted patients or when combined with other vasodilators
  • Reflex tachycardia – baroreceptor-mediated response to vasodilation; can worsen angina by increasing oxygen demand
  • Syncope – from orthostatic hypotension, especially with sublingual use in upright position

Neurological

  • Headache – very common (up to 50%), due to cerebral vasodilation; often decreases with continued use; can be severe and throbbing

Other

  • Flushing
  • Dizziness and lightheadedness
  • Nausea
  • Contact dermatitis – with transdermal patches
  • Methemoglobinemia – rare, occurs with very high doses or prolonged IV infusion; presents with cyanosis unresponsive to oxygen

Tolerance

Nitrate tolerance develops with continuous exposure, resulting in diminished therapeutic response. This is mediated by depletion of sulfhydryl donors, oxidative stress, and endothelial dysfunction. A nitrate-free interval restores efficacy.

Rebound angina

Abrupt discontinuation after prolonged use may cause rebound coronary vasospasm and angina, particularly with long-acting formulations.

Contraindications and warnings

Absolute contraindications

  • Concurrent use of phosphodiesterase-5 (PDE-5) inhibitors (sildenafil, tadalafil, vardenafil, avanafil) – can cause life-threatening hypotension; contraindicated within 24 hours of sildenafil or vardenafil, 48 hours of tadalafil
  • Concurrent use of riociguat (guanylate cyclase stimulator)
  • Severe hypotension (systolic BP <90 mmHg)
  • Hypovolemia
  • Right ventricular infarction – RV is preload-dependent; reducing preload can cause severe hypotension and cardiovascular collapse
  • Hypertrophic obstructive cardiomyopathy – reduction in preload worsens outflow obstruction
  • Constrictive pericarditis or cardiac tamponade – heart is preload-dependent
  • Increased intracranial pressure – nitrates can worsen cerebral edema
  • Closed-angle glaucoma (relative)

Warnings

  • Inferior wall MI: Use with caution; may indicate RV involvement
  • Severe anemia: Worsens oxygen delivery
  • Hepatic impairment: May require dose adjustment
  • Pregnancy: Category C; use only if benefit outweighs risk

Important clinical note: Always assess for PDE-5 inhibitor use before administering nitroglycerin in acute settings. Patients may be reluctant to disclose erectile dysfunction medication use.

Drug interactions

Major interactions

  • PDE-5 inhibitors (sildenafil, tadalafil, vardenafil, avanafil) – synergistic vasodilation causes profound, potentially fatal hypotension; absolute contraindication
  • Riociguat – similar mechanism; severe hypotension
  • Alcohol – enhances vasodilation and hypotensive effects

Moderate interactions

  • Antihypertensives – additive hypotensive effects with beta-blockers, calcium channel blockers, ACE inhibitors, ARBs
  • Dihydroergotamine – nitroglycerin may reduce the efficacy of dihydroergotamine for migraines
  • Heparin – nitroglycerin may decrease heparin effect (IV administration)
  • Alteplase – nitroglycerin may reduce thrombolytic efficacy

Beta-blocker combination

Beta-blockers are often co-administered with nitroglycerin for angina. Beta-blockers blunt reflex tachycardia caused by nitrates, improving the therapeutic profile. However, this combination increases risk of hypotension.

Aspirin

Aspirin may enhance nitrate levels and effects, which can be therapeutically beneficial in acute coronary syndromes but may increase adverse effects.

Clinical pearls

Exam and clinical high-yield points

  • Sublingual administration: Instruct patients to sit or lie down before taking sublingual nitroglycerin to prevent syncope. The tablet should tingle; lack of tingling may indicate expired medication.
  • Storage: Nitroglycerin is volatile and degrades with light and heat exposure. Keep tablets in original amber glass container, tightly closed. Replace every 3-6 months after opening.
  • Reflex tachycardia: The heart rate increase is a classic adverse effect and can be blunted by concurrent beta-blocker use.
  • Nitrate-free interval: Essential for long-acting formulations (patches, sustained-release oral). Typically remove patch at night or schedule oral doses to allow 10-14 hours nitrate-free to prevent tolerance.
  • Methemoglobinemia: Rare but serious; consider in patients receiving high-dose IV nitroglycerin who develop unexplained cyanosis and low oxygen saturation unresponsive to supplemental oxygen. Treat with methylene blue.
  • Topical nitroglycerin for anal fissures: Applied as 0.4% ointment to promote local vasodilation and healing. Headache is a common limiting side effect.
  • IV nitroglycerin: Requires special non-PVC tubing due to drug adsorption. Titrate carefully to blood pressure response.
  • Distinguishing angina from MI: If sublingual nitroglycerin relieves chest pain within minutes, it suggests stable angina. Pain unrelieved after initial administration warrants immediate evaluation for MI.

Common pitfalls

  • Forgetting to ask about PDE-5 inhibitor use in patients presenting with chest pain
  • Not recognizing right ventricular infarction as a contraindication (look for inferior MI with hypotension, clear lungs, elevated JVP)
  • Failing to implement a nitrate-free interval, leading to tolerance
  • Using expired nitroglycerin tablets
Mnemonic

NITRATES adverse effects: Nausea, Increased heart rate (reflex tachycardia), Tachyphylaxis (tolerance), Reflex sympathetic activation, Arterial dilation (hypotension), Throbbing headache, Erythema/flushing, Syncope.

Practice questions

1. A 62-year-old man presents to the emergency department with crushing substernal chest pain radiating to his left arm for the past 45 minutes. ECG shows ST-segment elevations in leads II, III, and aVF. Blood pressure is 88/54 mmHg, heart rate is 110 bpm, and jugular venous pressure is elevated. Lung examination reveals clear breath sounds bilaterally. Which of the following is contraindicated in this patient?

Answer: C. This patient has an inferior wall MI (ST elevations in II, III, aVF) with hypotension, elevated JVP, and clear lungs—classic for right ventricular infarction. Nitroglycerin is contraindicated because the RV is preload-dependent; reducing preload with nitrates causes severe hypotension and cardiovascular collapse. The correct management includes IV fluids to maintain preload. Aspirin and reperfusion therapy remain indicated.

2. A 58-year-old man with chronic stable angina presents for medication review. He uses sublingual nitroglycerin as needed and wears a nitroglycerin transdermal patch continuously. He reports that the medication is no longer as effective at relieving his chest pain. Which of the following is the most likely explanation for his symptoms?

Answer: B. Continuous use of nitroglycerin patches without a nitrate-free interval leads to tolerance, characterized by diminished therapeutic response. A 10-14 hour daily nitrate-free period (typically overnight) restores efficacy. While CAD progression is possible, the history of continuous patch use makes tolerance the most likely explanation. Expired tablets would not explain patch ineffectiveness, and MI would present more acutely.

3. A 67-year-old man with unstable angina is admitted to the coronary care unit and started on intravenous nitroglycerin. On hospital day 2, he develops cyanosis and his oxygen saturation drops to 85% despite high-flow oxygen. Arterial blood gas shows PaO2 of 110 mmHg. Blood appears chocolate-brown. Which of the following is the most appropriate immediate treatment?

Answer: B. This patient has methemoglobinemia, a rare complication of high-dose IV nitroglycerin. Key features include cyanosis unresponsive to oxygen, normal PaO2 (oxygen dissolved in plasma is normal), and chocolate-brown blood. Methylene blue is the antidote; it reduces methemoglobin back to hemoglobin. Increasing oxygen or ventilation will not help because the problem is hemoglobin's inability to carry oxygen, not inadequate oxygenation or ventilation.

Frequently asked questions

How does nitroglycerin work?

Nitroglycerin is converted to nitric oxide, which activates guanylate cyclase in vascular smooth muscle, increasing cyclic GMP and causing vasodilation. It preferentially dilates veins over arteries, reducing venous return (preload) and decreasing myocardial oxygen demand. This mechanism rapidly relieves angina by rebalancing oxygen supply and demand in the heart.

What are the side effects of nitroglycerin?

The most common side effects are headache (occurs in up to 50% of patients due to cerebral vasodilation), dizziness, lightheadedness, and flushing. Cardiovascular effects include hypotension, reflex tachycardia, and syncope, especially when standing. Tolerance develops with continuous use, reducing effectiveness. Rare but serious effects include severe hypotension (particularly when combined with PDE-5 inhibitors) and methemoglobinemia with high doses.

Why can't you take nitroglycerin with Viagra?

Combining nitroglycerin with sildenafil (Viagra) or other PDE-5 inhibitors causes profound, potentially fatal hypotension. Both drugs increase cyclic GMP levels through different mechanisms—nitroglycerin increases production via nitric oxide, while PDE-5 inhibitors prevent its breakdown. The synergistic effect leads to excessive vasodilation. Nitroglycerin is absolutely contraindicated within 24 hours of sildenafil or vardenafil, and 48 hours of tadalafil.

How quickly does sublingual nitroglycerin work?

Sublingual nitroglycerin works within 1-3 minutes for acute angina relief, with effects lasting 30-60 minutes. The medication is absorbed directly through the oral mucosa, bypassing first-pass metabolism in the liver. Patients should sit or lie down before taking it to prevent syncope. If chest pain is not relieved after the initial administration, emergency care should be sought immediately as this may indicate myocardial infarction.

What is nitrate tolerance and how do you prevent it?

Nitrate tolerance is reduced therapeutic response that develops with continuous nitroglycerin exposure, caused by depletion of sulfhydryl donors, oxidative stress, and endothelial dysfunction. To prevent tolerance, a nitrate-free interval of 10-14 hours daily is essential. For transdermal patches, remove at night; for sustained-release oral formulations, schedule doses to allow an overnight nitrate-free period. This rest period restores drug sensitivity and maintains clinical effectiveness.

Conditions it is used for

Angina pectorisAcute coronary syndromeUnstable anginaAcute decompensated heart failureMyocardial infarctionHypertensive emergencyEsophageal spasmAnal fissure
isosorbide dinitrateisosorbide mononitratehydralazinesildenafilmetoprololdiltiazemranolazineatenolol

Sources and further reading

DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: C01DA02WHO ATC/DDD index: C05AE01

Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.