Enteral

In pharmacology, Enteral (Greek, enteros : Intestine) is a term used to describe routes of drug administration that involve absorption of the drug through the gastrointestinal tract. The term may also be used to describe something as residing in the gastrointestinal tract. This is contrasted with the term parenteral (greek para: Besides + enteros: Intestine), meaning outside the intestines, or more specifically in pharmacology: Methods of introducing drugs or substances into the body other than through the gastrointestinal tract, via, for example, intravenous injection.

Forms of enteral administration
Enteral administration may be divided into two different categories, oral and rectal.

Drug absorption from the intestine
The mechanism for drug absorption from the intestine is for most drugs passive transfer, a few exceptions include levodopa and fluorouracil, which are both absorbed through carrier-mediated transport. For passive transfer to occur, the drug has to diffuse through the lipid cell membrane of the epithelial cells lining the inside of the intestines. The rate at which this happens is largely determined by two factors: Ionization and lipid solubility.

Factors influencing gastrointestinal absorption:
 * Gastrointestinal motility.
 * Splanchnic blood flow.
 * Particle size and formulation.
 * Physicochemical factors.

First pass metabolism
Drugs given by enteral administration may be subjected to significant first pass metabolism, and therefore, the amount of drug entering the systemic circulation following administration may vary significantly for different individuals and drugs.