Intravenous refers to administering the medications, fluids, or nutrients directly into the veins of a person.
An intramucular injection is when the substance is inyjected into a muscle, which has a lot of blood vessels and allows fast absorption of the substance This icommonly s used for vaccines andisthe common places of application are the gluteus nand the upper arm
Subcutaneous injections administer substances in the hypodermis, below the dermis. This tissue has few blood vessels, so is useful o r slow constant absorption. Infulin is an example of a substance delivered this way.