Nitroglycerin: How It Works, Uses & Contraindications

Nitroglycerin is the fast-acting nitrate that relieves angina within minutes. Here's how it works, the ways it's given, and the contraindications that can make it dangerous.

What is nitroglycerin?

Nitroglycerin is a nitrate vasodilator used to relieve and prevent angina — the chest pain of reduced coronary blood flow. It's one of the oldest and most reliable cardiac drugs, working within minutes when placed under the tongue.

How it works

Nitroglycerin is converted to nitric oxide, which relaxes vascular smooth muscle. The main effect is venodilation, which pools blood in the veins and reduces the amount returning to the heart (preload). Less preload means the heart does less work and needs less oxygen. Nitroglycerin also dilates the coronary arteries, improving supply and relieving spasm.

Forms and uses

Form	Typical use
Sublingual tablet / spray	Acute angina — fast relief
Intravenous infusion	Acute coronary syndromes, hypertensive emergency, acute heart failure
Transdermal patch / ointment	Long-term angina prevention

For sublingual use in angina, the usual advice is one dose repeated up to three times a few minutes apart — and to seek emergency help if pain persists, since it may be a heart attack.

Contraindications (important)

• PDE5 inhibitors (sildenafil, tadalafil) — combining them with nitroglycerin can cause life-threatening hypotension. Always ask.
• Right ventricular (inferior) infarction — these patients are preload-dependent and can crash with nitrates.
• Severe hypotension or hypovolemia.
• Severe aortic stenosis — falling preload can drop cardiac output.

Side effects & tolerance

The commonest side effects are a throbbing headache, flushing, dizziness, and low blood pressure. With continuous use the body develops nitrate tolerance, so patch or long-acting regimens build in a daily nitrate-free interval to keep them working.

Key takeaways

• Nitroglycerin relieves angina mainly by cutting preload (venodilation) and dilating coronaries.
• Forms: sublingual (fast), IV (acute), and patch/ointment (prevention).
• Never give with PDE5 inhibitors or in right-ventricular infarction.
• Headache and hypotension are common; continuous use causes tolerance.

Frequently asked questions

What is nitroglycerin used for?

To relieve and prevent angina (chest pain from reduced coronary blood flow), and intravenously in acute coronary syndromes, hypertensive emergencies, and acute heart failure.

How does nitroglycerin work?

It's converted to nitric oxide, which dilates veins to reduce preload (so the heart works less) and dilates coronary arteries to improve blood supply and relieve spasm.

When should you not take nitroglycerin?

Avoid it with PDE5 inhibitors like sildenafil (risk of severe hypotension), in right-ventricular/inferior infarction, in severe hypotension, and in severe aortic stenosis.

What are the side effects of nitroglycerin?

Headache, flushing, dizziness, and low blood pressure; continuous use can cause nitrate tolerance.

How do you take sublingual nitroglycerin for angina?

Place a tablet or spray under the tongue at the onset of chest pain; it can usually be repeated up to three times a few minutes apart, and you should seek emergency help if the pain does not settle.