Bioavailability

The proportion of a dose that actually reaches the bloodstream unchanged.

Bioavailability is the fraction of an administered dose that reaches the circulation in an active form. An intravenous dose is by definition fully bioavailable; a swallowed tablet is not, because part of it may fail to dissolve, fail to cross the gut wall, or be broken down by the liver before it ever reaches the rest of the body. This is why the oral and injected doses of the same drug are often very different numbers, and why food can meaningfully change how strong a dose feels.

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