In pharmacokinetics, bioavailability describes what?

• A. The time it takes for a drug to start having an effect.
• B. The fraction of a drug that reaches systemic circulation and is available to have an effect.
• C. The maximum concentration of a drug in plasma after administration.
• D. The rate at which a drug is eliminated from the body.

Answer: (B)

Biavailability is about how much of the administered drug actually reaches the bloodstream in an active form to produce an effect. It’s the fraction of the dose that becomes systemically available after absorption (and any first-pass metabolism). That is why this option is the best description: it directly states the portion of the dose that makes it into circulation and is able to have an effect.

In contrast, the time to onset refers to when the drug starts working, not how much enters circulation. The maximum plasma concentration describes peak exposure, not overall availability. The rate of elimination concerns how quickly the drug is removed from the body, not how much initially becomes available systemically.