Overview
Nitroglycerin is a fast-acting nitrate medication used primarily to treat and prevent angina (chest pain) caused by coronary artery disease. It works rapidly by relaxing and widening blood vessels, allowing more blood and oxygen to reach the heart muscle.

Mechanism of Action
It is converted to nitric oxide (NO) in the body, which activates guanylate cyclase, leading to an increase in cyclic GMP (cGMP) within vascular smooth muscle cells. This results in the relaxation of smooth muscle, causing vasodilation of veins and arteries. Venodilation reduces venous return (preload), while arterial dilation reduces vascular resistance (afterload), decreasing the heart’s workload and oxygen demand.

Dosage and Administration
For acute angina attacks, sublingual tablets (usually 0.3 to 0.6 mg) are placed under the tongue at the first sign of chest pain. If pain persists, a dose can be repeated every 5 minutes for up to 3 doses. If pain is not relieved after 15 minutes, emergency medical attention is required. It is also available as sprays, patches, and intravenous formulations.

Side Effects
The most common side effect is a severe, throbbing headache due to rapid vasodilation. Other common side effects include dizziness, lightheadedness, flushing, and a drop in blood pressure, especially when standing up quickly (orthostatic hypotension).

Contraindications
It is strictly contraindicated in patients taking phosphodiesterase type 5 (PDE5) inhibitors (e.g., sildenafil, tadalafil) for erectile dysfunction, as the combination can cause a life-threatening drop in blood pressure. It is also contraindicated in patients with severe anemia, increased intracranial pressure, or known hypersensitivity to nitrates.

Pregnancy and Lactation
It should be given to a pregnant woman only if clearly needed, as safety during pregnancy has not been fully established. It is not known whether nitroglycerin is excreted in human milk, so caution should be exercised when administered to a nursing woman.