infusion
Where an injection is a quick shot, an infusion is a slow pour. It is a large volume of sterile fluid — often hundreds of millilitres or more — fed gently into a vein over minutes or hours, like topping up a tank rather than splashing in a cupful all at once.
Infusions serve two broad purposes. They replace fluid, salts and energy when a patient cannot eat or drink (saline, glucose and electrolyte solutions, or nutrition delivered through a vein). And they act as a slow, steady delivery channel for drugs, where a small dose of medicine is added to a large bag and the rate of drip controls how fast the drug enters the bloodstream.
Because so much liquid enters the circulation directly, infusions must be especially clean: sterile, free of pyrogens, and matched to blood tonicity so that running them in does not damage red cells or overload the heart. The flow is governed by gravity through a drip set or, more precisely, by an infusion pump.
A practical caveat: infusing too fast can flood the circulation or cause adverse reactions, while infusing too slowly delays therapy. Getting the rate right is as much a part of the dose as the amount in the bag.
A 500 mL bag of 0.9% sodium chloride hung at the bedside and dripped in over four hours is an infusion; adding a dose of an antibiotic to that bag turns it into a vehicle for slow drug delivery.
Large volume, slow rate, into a vein — and the drip rate is part of the prescription.
Infusions are almost always large-volume parenterals and almost always intravenous. Because the dose is spread over time, a steady plasma drug concentration can be held — useful for drugs with a narrow safety window.