subcutaneous

Subcutaneous refers to the tissue layer beneath the dermis of the skin and to injections delivered into that layer. In humans this layer, also called the hypodermis, consists mainly of loose connective tissue and adipose (fat) tissue, with a network of blood vessels and nerves. It lies between the dermis and the underlying muscle or fascia, and its thickness varies by body site, sex, age, and body fat.

Anatomy and function: The subcutaneous layer acts as insulation, cushioning, and a storage site for fat. It provides a relatively loose, highly vascularized path for small nerves and vessels from the skin to deeper tissues. Its structure varies across individuals and sites, which can influence how medications are absorbed when given subcutaneously.

Medical relevance and administration: Subcutaneous injections deliver medications into this tissue. Absorption is generally slower than intramuscular injections due to lower tissue perfusion, yielding gradual onset and longer duration for certain drugs. This route is chosen for drugs that benefit from slow uptake and for some vaccines and biologics. Common examples include insulin, heparin, and several vaccines or biologic therapies, though the suitability of subcutaneous administration depends on the product.

Sites, technique, and safety: Common injection sites are the abdomen (avoiding the navel area), the outer upper arm, and the front of the thigh; sites are typically rotated to reduce tissue changes such as lipodystrophy. Needle length and angle depend on patient size and device, but the goal is to reach subcutaneous tissue rather than muscle. Proper hand hygiene and site preparation reduce infection risk. Complications can include local skin reactions, bleeding, and lipodystrophy with long-term use.