- For the mathematical term, see Injection (mathematics).
An injection is a liquid injected into the body. It is typically administered by one of three routes: intravenous (directly into the bloodstream), intramuscular (into one of the muscles, typically the deltoid or gluteus), or subcutaneous (into the fatty tissue underneath the skin, typically used for anticoagulants such as heparin). There are other routes of administration, such as intraventricular (directly into the brain via an Ommaya reservoir), or intrathecal (directly into the spinal cord, typically for anesthesia). It is frequently used for medicinal purposes, though also used illicitly to self-administer certain illegal drugs. The injection itself is given with some model of a hypodermic needle; or through a previously inserted catheter, be it intravenous, intrathecal, or intraventricular.