Subcutaneous vs. Subdermal — What's the Difference?

Subcutaneous injections are administered into the layer of fat and connective tissue beneath the skin, which has fewer blood vessels, making it an ideal route for medications that require slow, consistent absorption. On the other hand, subdermal (also known as intradermal) injections target the dermis, the layer of skin between the epidermis and subcutaneous tissues. This method is used for diagnostic purposes, like allergy tests and tuberculosis screening, because the dermis has a rich blood supply that can elicit a rapid immune response.

Subcutaneous injections are preferred for vaccines and medications like insulin, due to the slow, steady absorption from the subcutaneous fat layer. Whereas, subdermal injections are often used for sensitivity tests, such as the Mantoux test for tuberculosis, because the dermis layer reacts visibly to foreign substances.

The technique for administering subcutaneous injections involves pinching the skin to create a fold and inserting the needle at a 45 to 90-degree angle, ensuring the medication is deposited into the fatty layer. In contrast, subdermal injections require a shallower angle, usually 10 to 15 degrees, with the needle bevel facing up, to place the substance into the skin itself.

Given the distinct layers targeted, subcutaneous and subdermal injections also differ in their potential side effects and complications. Subcutaneous injections can sometimes lead to lipodystrophy (changes in fat tissue) at injection sites, while subdermal injections might cause more localized reactions like redness or swelling.

The choice between subcutaneous and subdermal injections depends on the medication, its desired effect, and the required absorption rate. Subcutaneous injections are generally used for long-term therapies, such as insulin for diabetes, due to their slower absorption, while subdermal injections are chosen for immediate immune response needs, like allergy tests.