route of administration
The route of administration is simply the doorway through which a drug enters the body — by mouth, by injection into a vein or muscle, under the tongue, through the skin, into the lungs by inhalation, and many others. Choosing the door is not a minor detail: it shapes how fast the drug acts, how much of it gets in, and how convenient and safe the treatment is.
Routes split broadly into enteral, going through the gut (most importantly swallowing), and parenteral, bypassing the gut (such as intravenous, intramuscular or subcutaneous injection). The gut route is convenient but exposes the drug to incomplete absorption and first-pass metabolism, while intravenous delivery skips those losses entirely, placing the full dose directly in the blood with immediate effect.
The right route depends on the drug's properties and the clinical need. A molecule destroyed in the gut or too poorly absorbed to work orally may have to be injected or inhaled; an emergency calls for the speed of an intravenous line; a chronic, stable condition favors the convenience of a daily tablet. Route therefore interacts tightly with formulation and with the chemistry of the molecule itself.
Nitroglycerin is placed under the tongue rather than swallowed: the sublingual route lets it enter the blood directly through the mouth's lining, avoiding the heavy first-pass metabolism that would destroy most of an oral dose.
Choosing the route can sidestep first-pass loss.
Only the intravenous route is defined as 100 percent bioavailable, since it deposits the drug straight into the blood; every other route is measured against it and loses some fraction along the way.