Medical injector is the use of a needle and a syringe to put a liquid, especially a drug, into the body. Injection is a technique for delivering a drug by parenteral administration, that is, administration by a route other than the digestive tract. Parenteral injections include subcutaneous, intramuscular, intravenous, intraperitoneal, intracardiac, intra-articular and intracavernous injections.

Medical injector is usually administered in the form of a bolus, but can also be used for continuous administration. Even if administered by bolus injection, the drug may be long-acting and then called long-acting injection. In the case of longer term or relapse drug administration, it is generally preferred to administer via an indwelling catheter rather than an injection.

Injection is one of the most common health care programs, managing at least 16 billion annually in developing and transition countries. 95% of the injections are used for therapeutic care, 3% for immunization, and the rest for other purposes, such as blood transfusions. In some cases, the term injection is used synonymously with different workers in the same hospital.