bioavailability
Bioavailability answers a simple question: of the dose you swallowed (or injected into muscle, or stuck on the skin), how much actually made it into the bloodstream intact and ready to work? Imagine pouring water into a leaky bucket — bioavailability is the fraction that survives the leaks. It is written as F and ranges from 0 to 1 (or 0–100%).
By definition, an intravenous dose has a bioavailability of 1 (100%), because it is placed directly into the circulation with nothing lost. Other routes are compared against this benchmark. Oral drugs commonly lose some of their dose to incomplete absorption across the gut wall and to first-pass metabolism, where the liver chemically degrades part of the dose before it reaches the rest of the body.
Absolute bioavailability compares a route to intravenous dosing; relative bioavailability compares two non-IV formulations. A low or highly variable F is a practical headache: it means more drug must be given to achieve the same effect, and the response can swing unpredictably between patients or between meals.
Oral morphine has a bioavailability of only about 25% because of heavy first-pass metabolism, so an oral dose must be several times larger than an intravenous one to give a comparable effect.
First-pass loss makes oral morphine's F low.
Oral bioavailability = (fraction absorbed) × (fraction surviving first-pass). A drug can be fully absorbed yet have low F if the liver destroys most of it on the first pass.