The obstacle course of an oral dose
Swallow a tablet and the molecule faces three hurdles before it counts as 'absorbed.' First it must dissolve in the watery contents of the gut — no solubility, no absorption. Then it must cross the intestinal wall, a lipid membrane, which demands the right permeability. Finally, blood from the gut goes straight to the liver, where some of the drug may be destroyed before it reaches general circulation — the first-pass effect.
What bioavailability really means
Bioavailability (often written F) is the *fraction of an administered dose that reaches the systemic circulation unchanged*. By definition, an intravenous (IV) dose has F = 100% — it is injected straight into the blood. Oral bioavailability is measured against that IV benchmark, and it is the product of three fractions: the fraction absorbed, the fraction surviving the gut wall, and the fraction surviving first-pass liver metabolism.
F (oral) = f_abs × f_gut × f_hep Example: f_abs = 0.90 (90% dissolves and is taken up) f_gut = 0.95 (5% destroyed in gut wall enzymes) f_hep = 0.50 (half destroyed on first pass through liver) F = 0.90 × 0.95 × 0.50 = 0.43 -> ~43% oral bioavailability Even with near-perfect absorption, heavy first-pass metabolism alone can cap F below 50%.
Tuning absorption by design
Decades of data gave us the famous rule of five: orally absorbed drugs *tend* to have molecular weight under 500, no more than 5 H-bond donors, 10 acceptors, and moderate lipophilicity. It is a guideline, not a law — many fine drugs break it — but it captures the physics of a molecule that can both dissolve and cross a membrane. In the lab, chemists predict permeability with the Caco-2 cell assay before ever dosing an animal.