Learn pharmacokinetics
Bioavailability: how much of a dose reaches the blood
Medically reviewed by Javeria Muneer, Pharmacist · Last reviewed:
Bioavailability, written F, is the fraction of an administered dose that reaches the systemic circulation unchanged. An intravenous dose has F = 1 by definition. Oral doses are usually lower because some drug is never absorbed and some is metabolised by the gut wall and liver before it reaches the rest of the body, the first-pass effect.
Examples
Paracetamol and ibuprofen have high oral bioavailability, around 0.8 to 0.9. Morphine's is closer to 0.3, which is why oral doses are several times larger than injected ones. Vancomycin is barely absorbed at all, so oral vancomycin works only inside the gut.
What changes F
Food, stomach acid, other medicines, formulation and gut transit time all affect absorption. Antacids and dairy reduce ciprofloxacin absorption by binding to it. Grapefruit juice raises some drugs' bioavailability by blocking gut enzymes.
This is one reason a medicine bought abroad may not behave exactly like your usual brand even with the same active ingredient: different excipients and release mechanisms can change the rate and sometimes the extent of absorption.
See it in the simulator
Lower F from 0.9 to 0.3 and watch the whole curve shrink in proportion.
