Which route of administration involves injecting medication directly into major veins?

The route of administration that involves injecting medication directly into major veins is intravenous. This method allows for rapid delivery of medication into the bloodstream, providing immediate effects because it bypasses the absorption barriers encountered in other routes. Intravenous administration is particularly important in situations where quick therapeutic responses are needed, such as in emergency care, or when high concentrations of medication are required.

Other routes, while useful for specific situations, do not involve injecting directly into major veins. For example, intramuscular injections deliver medication into muscle tissue, which then has to be absorbed into the bloodstream, making the onset of action slower than intravenous administration. Subcutaneous injections involve administration into the fat layer beneath the skin, and absorption there is also relatively slow compared to intravenous methods. Intradermal injections place the drug just beneath the epidermis, often used for allergy tests or local anesthetics, but again do not provide the rapid systemic effect achieved through intravenous delivery.