intravenous administration

Intravenous (IV) administration puts a drug straight into a vein, so it enters the bloodstream immediately. It is the most direct route imaginable — there is no membrane to cross and no waiting for absorption, like pouring water directly into a river instead of letting it soak through soil first.

Because the entire dose lands in the blood, IV administration gives 100% bioavailability by definition; it is the reference standard against which all other routes are compared. The effect can begin within seconds to minutes, which makes IV essential in emergencies (cardiac arrest, severe infection, shock) and for drugs needing tight control. It also allows continuous infusion to maintain a steady blood concentration, and it can deliver large volumes and irritant solutions diluted in fluid.

The speed that makes IV powerful also makes it dangerous. An overdose or wrong drug cannot be retrieved once injected, and a too-rapid 'bolus' can spike blood levels to toxic peaks, causing heart rhythm problems or sudden collapse. IV access also risks infection, clot formation (thrombophlebitis), and air embolism, and it requires skilled placement of a cannula.

Note that IV drugs skip absorption entirely, but they do not skip metabolism or excretion — once in the blood, the liver and kidneys still process and remove them like any other drug.