What is the term used for a substance injected directly into a vein?

The term used for a substance injected directly into a vein is "intravenous." This method allows for the rapid delivery of medications, fluids, and nutrients directly into the bloodstream, providing immediate access to the circulatory system. Intravenous administration is commonly used in medical settings for various treatments, such as administering medications, hydration, or blood transfusions, as it ensures that the substances are quickly and effectively distributed throughout the body.

In contrast, subcutaneous refers to injections made into the fatty tissue just beneath the skin, intramuscular involves injecting substances directly into a muscle, and transdermal describes the delivery of medications through the skin via patches or creams. Each of these methods has its own specific uses and pharmacokinetics, but when it comes to delivering substances directly into the bloodstream, intravenous is the correct term.