if bioavailability is defined as "the rate and extent to which a drug reaches the systemic circulation"
Right, time to clear this up.
The definition of bioavailability is: "The fraction of the administered drug that ultimately enters the systemic circulation,
with the exception of the hepatic portal vein.
The hepatic portal vein is a large vein that carries blood from the small intestine, where nutrients and most drugs are absorbed, to the liver, where most drugs are metabolised and deactivated. This means that any drug that is taken orally and is not taken up in the stomach but in the small intestine will
first pass through the liver. Here a certain fraction of the drug is
metabolised before entering the bloodstream, i.e. first-pass metabolism.
Bioavailability is not an absorption rate and has nothing to do with how quickly or for how long a drug is active.
It is simply a measure of how much of the drug that you "take" ends up in your blood, from where it can start doing its job of making you happy.
("Take" = swallow, snort, inject, inhale, insert in various body cavities, drip into your eyes or just place under your tongue)
Having said all that, this lowly newb humbly begs alasdairms forgiveness for correcting an admin. Please don't smite me!
*looks around, surprised to be alive*
OK then. My dear Alasdairm, could you point me to somewhere I can find MDMA bioavailability following different ROAs? Pubmed, web of science, google, wiki and the BL 12-page bioavailability thread have all let me down.