Which term refers to a method of injecting substances into the muscle?

The term that refers to the method of injecting substances directly into the muscle is "intramuscular." This route of administration is used when a rapid systemic effect is desired, as muscles have a rich blood supply that facilitates the swift absorption of medications into the bloodstream. Intramuscular injections are commonly used for vaccinations, certain medications, and hormone therapies.

In contrast, the other methods listed serve different purposes. Subcutaneous injections involve delivering substances into the tissue layer between the skin and muscle, which may lead to slower absorption compared to intramuscular methods. Intravenous injections involve delivering substances directly into the bloodstream via a vein, allowing for immediate effects but requiring more skill and a different anatomical target. Intranasal refers to administration through the nasal passages, which is primarily suited for particular medications, emphasizing a different absorption route entirely. Each method has its specific applications based on the desired speed and type of drug absorption.