Which term refers to administration of medication directly into the muscle?

The term that refers to the administration of medication directly into the muscle is "Intramuscular." This method allows for the rapid absorption of the drug into the bloodstream due to the rich blood supply found in muscle tissue. Intramuscular injections are commonly used for various types of medications, including vaccines, antibiotics, and hormonal treatments, as they allow for a faster onset of action compared to other routes of administration.

In contrast, intravenous administration involves delivering medication directly into the bloodstream via a vein, providing immediate effects but requiring different techniques and equipment. Subcutaneous refers to medication injected into the tissue layer between the skin and muscle, which is generally absorbed more slowly than intramuscular injections. Per Os (or PO) signifies administration by mouth, which involves absorption through the digestive tract and typically takes longer for the medication to begin exerting its effects. Each of these methods has specific indications and uses in veterinary practice, but for direct injection into muscle, intramuscular is the appropriate terminology.