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An injection is the act of introducing a liquid into the body via a needle and syringe or other device. In medical contexts, injections deliver medicines, vaccines, or diagnostic agents. Injections differ from oral or topical administration because they bypass the skin and gastrointestinal tract, enabling rapid absorption or targeting certain tissues or compartments.

Common routes include intramuscular, subcutaneous, intravenous, and intradermal injections. Each route has typical needle sizes and injection depths. Other routes exist, such as intrathecal (spinal), intra-articular (joint) and epidural. Devices include disposable syringes, cartridge-based systems, auto-injectors, and jet injectors, the latter delivering the dose under pressure without a needle.

In clinical practice, injections are central to vaccination programs, pain management, diabetes care (insulin), sedation, anesthesia, and the delivery of many systemic and local therapies. Injections require aseptic technique, correct dosage, and consideration of patient-specific factors such as age, weight, and injection site. Safety includes proper disposal of sharps, avoiding needle reuse, and monitoring for adverse reactions.

The hypodermic syringe emerged in the 19th century, with contributions from Alexander Wood and Charles Pravaz, enabling more reliable and sterile injections. Since then, innovations include prefilled syringes, auto-injectors for rapid emergency administration, and devices for needle-free delivery. Injections remain a fundamental method in medicine and veterinary care, with ongoing advances in devices and routes to improve safety, efficacy, and patient comfort.