Injection site reactions (ISRs) are reactions that occur at the site of injection of a drug. They may be mild or severe and may or may not require medical intervention. Some reactions may appear immediately after injection, and some may be delayed. Such reactions can occur with subcutaneous, intramuscular, or intravenous administration.

Drugs commonly administered subcutaneously include local anesthetics, drugs used in palliative care (e.g., fentanyl and morphine), and biopharmaceuticals (e.g., vaccines, heparin, insulin, growth hormone, hematopoietic growth factors, interferons, and monoclonal antibodies).

Some reactions, such as pain, may appear immediately. Others may be delayed, such as erythema which may appear 24–96 hours after injection.

ISRs commonly seen with subcutaneous injections include:

Severe reactions may result in cutaneous necrosis at the injection site, typically presenting in one of two forms: (1) those associated with intravenous infusion or (2) those related to intramuscular injection. Intramuscular injections may produce a syndrome called livedo dermatitis.

There are many factors that can affect incidence of injection site reactions. They may be related to the drug formulation itself, to the method of injection, or to the patient.

Some factors such as volume of injection and speed of injection seem to not be well correlated with incidence of reaction.

Features of the needle used for injection can affect ISRs: