Nitrates are drugs that act as vasodilators, which means that they can increase the calibre of blood vessels, thereby increasing the flow of blood through them. In an angina attack, the patient doesn’t have much time, and therefore sublingual nitrates are given. Unlike the usual oral route of most drugs, sublingually administered drugs act within a few minutes as the blood vessels on the undersurface of the tongue transport the drug directly into the bloodstream, bypassing the usual long route of the digestive system and liver, which also reduces the concentration of the drug through a mechanisms known as first-pass metabolism.
The first sublingual tablet of nitrates is taken as soon as the attack starts and an additional two tablets may be taken at 5 minutes intervals. If no improvement is noticed, it may point towards an impending heart attack, and seeking medical care becomes a must. One of the most common side effects of nitroglycerin and nitrates, in general, is that they are not so specific to the heart, and generalized vasodilation may occur. This leads to hypotension, especially what we call “orthostatic hypotension”. This condition involves “pooling” of blood in the legs, and therefore deprives the brain of a part of its blood supply suddenly, leading to loss of consciousness. That’s why you should always take nitrates while sitting.