An injection is a method of putting liquid into the body with a hollow needle and a syringe which is pierced through the skin to a sufficient depth for the material to be forced into the body. An injection follows a parenteral route of injection, that is, its effect is not necessarily local to the area in which the injection is administered; it is systemic.

There are several types of injections or infusions, including, subcutaneous, intramuscular, and intravenous infusions. Long-acting forms of subcutaneous/intramuscular injections are available for various drugs; these are called depot injections.