In a way that goes through a vein, into the blood. You use it to tell how medicine is given: not as a pill you swallow, and not as a shot in the muscle, but through a needle into a vein. A sentence like “The drug is given intravenously” is typical in hospital and medicine writing. Everyday English more often says “through an IV” or “into a vein.” Keep it for the how — it is not the name of the tube or the bag itself.