[Lotus]

Thanks Lotus.

One process results in a reduced amount of drug that the body actually ends up utilizing; the other results in an increase in the amount of drug that the body ends up utilizing. Here's how:

First pass metabolism is what occurs when a drug is absorbed from the GI tract. When a drug is taken orally it is absorbed into the portal circulation (the blood vessels of the liver). Many of these drugs are very efficiently metabolized (altered for elimination) as they pass through the portal circulation during this first time. It reduces the amount of active drug that gets into the general circulation. This is first-pass metabolism.

Enterohepatic cycling is where:
Unmetabolized drugs as well as drug metabolites go through the liver and biliary tract for excretion and proceed to make their way out of the body through the intestinal tract. In other words, this is the body's way of putting them in the trash and getting rid of them.
Here's the catch. . .on the way out through the intestines (the entero part of the word enterohepatic) some of the discarded active drug gets reabsorbed back into the blood stream where it is again available to the body for use. In other words, it's being recycled.
RESULT: The half life and duration of action of a drug is increased.

First-pass elimination. Basic concepts and clinical consequences.
http://www.ncbi.nlm.nih.gov/pubmed/6362950

http://wikipedia.org/wiki/First_pass_effect

Drug metabolism
http://en.wikipedia.org/wiki/Drug_metabolism