The four main intramuscular injection sites are the upper arm, thigh, buttocks, and hip. Learn about other types of injections and how they are performed. An injection is a medical procedure that involves administering a liquid substance, such as a medication, vaccine, or therapeutic agent, directly into the body using a needle and syringe.

Injections are classified in multiple ways, including the type of tissue being injected into, the location in the body the injection is designed to produce effects, and the duration of the effects.