DrugTheGuide
PHARMACOKINETICS · ABSORPTION

Oral bioavailability

also: F, absolute oral bioavailability, systemic bioavailability

The fraction of an orally administered dose that reaches systemic circulation in unchanged, active form. It integrates intestinal absorption, gut wall metabolism, and hepatic first-pass extraction. A drug with F = 0.80 (80%) means that 80% of the oral dose reaches the bloodstream; the remaining 20% is lost to poor absorption or pre-systemic metabolism.

Formula and measurement

F = fa × fg × fh
fa = fraction absorbed from GI lumen (0–1)
fg = fraction escaping gut wall metabolism (CYP3A4, P-gp)
fh = fraction escaping hepatic first-pass metabolism
Measurement: F = AUCpo / AUCIV × DoseIV / Dosepo
Vancomycin (oral)
<5% — not systemically absorbed
Lidocaine (oral)
~3% — 97% first-pass hepatic
Ibuprofen (oral)
~80–90% — well-absorbed, low first-pass
Morphine (oral)
~20–40% — significant first-pass

BCS classification and bioavailability impact

BCS Class Solubility Permeability F (general) Key strategy
Class I High High High and consistent Simple formulation; biowaiver eligible
Class II Low High Variable; dissolution-limited Amorphous dispersion, lipid formulation, nanosizing
Class III High Low Variable; P-gp/transporter limited Increase permeation; inhibit efflux transporters
Class IV Low Low Generally poor and variable Complex formulation needed; high development risk

Frequently asked questions

What is oral bioavailability (F)?
Oral bioavailability (F) = fraction of oral dose reaching systemic circulation unchanged. F = fa × fg × fh, where: fa = fraction absorbed from GI lumen; fg = fraction escaping gut wall metabolism (CYP3A4/P-gp in enterocytes); fh = fraction escaping hepatic first-pass metabolism. F is measured by comparing AUCoral to AUCIV: F = AUCpo/AUCIV × doseIV/dosepo. Range: 0 (not absorbed at all) to 1.0 (100%). Compare with our bioavailability glossary entry and AUC.
What factors reduce oral bioavailability?
Factors reducing F: 1. Poor solubility: BCS Class II/IV drugs (lipophilic, poorly soluble) — addressed by nanomilling, amorphous dispersions; 2. Acid instability: penicillin G, omeprazole (enteric-coated to protect); 3. P-gp efflux: P-glycoprotein in enterocytes pumps drug back into lumen — inhibitable by grapefruit juice furanocoumarins, ketoconazole; see drug transporters guide; 4. Gut CYP3A4: cyclosporin, midazolam, felodipine — dramatic grapefruit interaction increases F by inhibiting intestinal CYP3A4; 5. First-pass hepatic metabolism: lidocaine F~3%, morphine ~35%, propranolol ~25% — see first-pass effect; 6. Disease: short bowel syndrome, IBD, coeliac disease reduce fa. Reference: NIH pharmacokinetics ↗.
How does the BCS (Biopharmaceutics Classification System) relate to bioavailability?
BCS classifies drugs by solubility (dose/solubility volume ≤250 mL at pH 1–7.4) and permeability (Peff ≥2×10−4 cm/s, or Caco-2 ≥100 nm/s): Class I (high/high): high F, complete absorption — FDA BCS-based biowaivers ↗. Class II (low/high): dissolution-limited F — improved by amorphous solid dispersions, lipid formulations, nanosizing. Class III (high/low): permeability-limited — P-gp substrates common. Class IV (low/low): poor, variable F — formulation challenge. BCS is used by EMA ↗ and FDA to grant biowaivers (Class I/III) — allows generic approval without clinical PK studies if dissolution criteria are met. Related: bioequivalence.
What is relative bioavailability and how does it differ from absolute bioavailability?
Absolute F: AUCpo/AUCIV (IV = reference; 100% available). Relative F (Frel): AUCtest/AUCreference comparing two non-IV formulations. Bioequivalence criteria: AUC and Cmax 90% CI within 80–125% of reference (FDA/EMA standard). Pharmacokinetic studies: crossover design measures AUC0-∞, Cmax, tmax. High intra-subject variability (ISV) drugs may require replicate design studies (reference-scaled AUC). Food effect studies (fed vs fasted): required for most oral BLA/NDA submissions. See FDA pharmacokinetics guidance documents ↗ and Cmax glossary.
Which drugs have notably low or high oral bioavailability and why?
Very low F (high first-pass/non-absorbed): Vancomycin F <5% po — used PO only for C. difficile colitis (local GI action); Lidocaine F ~3% — 97% first-pass hepatic; GTN/nitroglycerin F <2% oral (sublingual or transdermal bypasses portal circulation); insulin F <1% (protease degradation — see insulin guide). High F: Amlodipine ~64–90%, metformin ~50–60%, ibuprofen ~80–90%. Variable F due to genetics: CYP2D6 polymorphisms affect codeine→morphine conversion and tamoxifen→endoxifen conversion — pharmacogenomics guide. Grapefruit juice dramatically increases F of cyclosporin, midazolam, statins (CYP3A4/P-gp inhibition). See CYP450 guide.