What does the "bioavailability" of a drug refer to?

Pharmacokinetics basically asks the question, "Is the drug getting to its site of action?" It is usually considered under four headings (acronym: ADME):

• Absorption (how much drug is available to the body and how fast)
• Distribution (where the drug goes within the body)
• Metabolism (how the body eliminates the drug via chemically 'neutering' it)
• Excretion (how the body eliminates the drug, or its metabolite, by physically removing it from the body - e.g. in urine or faeces)

"Bioavailability" falls under the absorption arm of pharmacokinetics. It's basically a measure of how much of the drug becomes available in the body to work its magic. We tend to use this term instead of "absorption" when talking about a particular drug's properties, but only because it's much easier to measure. 'Bioavailability' includes absorption, but it also takes into account any metabolism that takes place before the drug enters the systemic circulation. To illustrate this, consider a tablet that you swallow, in the hope that it relieves your joint pain.

Clearly, the tablet can only have an effect if it is absorbed from the gastrointestinal tract (GIT) into the blood stream. However, this is not enough by itself: the blood from the GIT passes through the liver and then the lungs before it is available to the rest of the body (via the systemic circulation). Thus, for a drug to work, not only must some of it be absorbed, but at least some of it must reach the site of action (e.g. your painful joints) without being metabolised by the GIT, liver, or lungs.

Now, if you take a blood level for your drug, all you have an idea of is how much of the drug is available to the body. It can be very hard to determine how much of the drug was lost through not being absorbed, versus how much of the drug was lost through pre-systemic metabolism. Therefore, we usually talk simply of "bioavailability" (how much of the drug is available to the systemic circulation) rather than absorption, since we don't know exactly how much WAS absorbed in the first place.