Bioavailability
Say it: by-oh-uh-vay-luh-BIL-ih-tee
Bioavailability is the fraction of a drug dose that reaches the general circulation unchanged, along with how quickly it gets there.
Definition
What is bioavailability?
Bioavailability is the share of a dose of a drug that actually reaches the general bloodstream in active form, from where it can be carried to its site of action. It is expressed as a fraction or percentage, given the symbol F, and also covers the rate at which the drug arrives. A drug injected into a vein counts as 100 percent bioavailable, because none is lost on the way in.
Other routes lose some of the dose. A swallowed tablet has to dissolve, cross the gut lining and pass through the liver before reaching the rest of the body, and enzymes in the gut wall and liver may break part of it down first, the first-pass effect. Poor solubility, stomach acid, binding to food or other drugs, and quick passage through the gut can lower bioavailability further. The antibiotic gentamicin is barely absorbed by mouth and must be injected.
Bioavailability is measured by plotting blood concentration against time after a dose and comparing the area under the curve with that after an intravenous dose. Two products of the same drug, such as a brand and a generic, are bioequivalent when their curves closely match, which matters most for drugs with a narrow therapeutic index.
Word parts
Where does the word “bioavailability” come from?
- bio-
- lifeGreek bios
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Related
Terms that go with it
- First-pass metabolismFirst-pass metabolism is the breakdown of an oral drug in the gut wall and liver before it reaches the general circulation, reducing how much survives.
- PharmacokineticsPharmacokinetics is the study of how the body absorbs, distributes, metabolizes and excretes a drug, and how drug levels change over time.
- Half-lifeHalf-life is the time it takes for the concentration of a drug in the blood, or its amount in the body, to fall by half.
- LiverThe liver is the largest gland and internal organ, lying in the upper right abdomen, where it makes bile, processes nutrients and clears toxins.
- Therapeutic indexThe therapeutic index is a ratio comparing the dose of a drug that causes toxicity with the dose that produces the desired effect.
Sources
Checked against
Written for biofideX in plain words, after checking these references:
- MSD Manual Professional: Drug Bioavailability
- OpenStax Pharmacology for Nurses, 2.2 Pharmacokinetics and Pharmacodynamics
- OpenStax Pharmacology for Nurses, 2.3 Drug Administration Routes, Preparation, and Administration
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