An Infusion is when fluids, medication, or blood is given through a catheter directly into a vein. A medical professional does this by using a needle to guide the catheter into a vein. The needle is then removed leaving the plastic catheter behind in the vein. An injection refers to the administration of medication using a needle and syringe. For example, if you’ve been given a flu shot or a vaccine, it was an injection. The big difference between an infusion and an injection is the period of administration. On the one hand, injections are often done within minutes. On the other hand, infusions can take anywhere between 30 minutes to several hours.