Nitroglycerin taken sublingually bypasses the first pass metabolism of liver and acts quickly. That is why, it is preferred in termination of an acute attack of angina where a rapid action is needed.
Nitroglycerin in low doses, causes venodilation, decreasing preload. In higher doses, it causes arterial and venous dilation, causing a decrease in preload and afterload.
Repeated and frequent exposure causes tissue tolerance to nitrates. That is why, nitrate free interval is recommended daily, especially with the use of transdermal patches or ointment.
Side effects of nitroglycerin are headache, postural hypotension and reflex tachycardia.