A subcutaneous injection is a method of administering a substance into the subcutaneous tissue, the layer of fat and connective tissue situated between the skin and underlying muscle. It is one of several parenteral routes, meaning routes that bypass the digestive tract.
This route is commonly used for peptide-based and other large-molecule compounds because it allows them to enter circulation without being broken down by digestive enzymes, as would occur if they were swallowed.
Subcutaneous delivery typically results in slower, more gradual absorption than intravenous administration. It is described here as a defined route of administration within formulation science.