This is based on the specific medication being used, the rate of absorption desired and the specific site of action (where the medication needs to have an effect).
Most drugs are manufactured for a specific route of administration and must be used as directed for safety and efficiency.
Some examples of routes of administration include:
Oral: Medications are taken by mouth (in pill, capsule or liquid form) and absorbed into the system through the digestive system. Absorption is slow. Medications that use this option cannot be used if vomiting is occurring.
Enteral: Medications are administered into the rectum (via a suppository) and absorbed by the lower digestive tract.
Mucosal: Medications are delivered through the nose or inhaled and are absorbed through the nasal mucosa or bronchioles, respectively. Vaginal administration of a medication is also considered mucosal.
Parenteral: Any medication administration that involves injecting a drug directly into a vein (intravenous), muscle (intramuscular), artery (intrarterial), abdominal cavity (intrperitoneal), heart (intracardiac) or into the fatty tissue beneath the skin (subcutaneous). The speed of absorption varies but is faster than oral administration. This is used when more complete and faster absorption of a drug is needed.
Percutaneous: Medications are absorbed directly through the skin into the blood stream. Some hormone replacements are administered by patches that are absorbed slowly and evenly, for example.