The correct choice, transdermal, is not classified as a type of injection. Instead, transdermal refers to the method of delivering medication through the skin for systemic absorption, often seen in patches that provide a slow release of medication over time.
In contrast, intradermal, subcutaneous, and intravenous are all recognized methods of administering injections. Intradermal injections are given into the dermis, just under the epidermis, typically used for allergy testing or vaccines. Subcutaneous injections are administered into the fatty tissue just below the skin, which allows for slower absorption compared to intramuscular or intravenous methods. Intravenous injections deliver substances directly into the bloodstream, providing immediate effects, often used in emergency situations or for fluid therapy. Each of these three methods involves breaking the skin barrier with a needle, a key characteristic that distinguishes them from transdermal administration.