Apart from the rate of a drug’s absorption, we also need to consider the completeness of its absorption. Bioavailability is used to report this and it is defined below as:

**Bioavailability: The proportion of an administered dose that is absorbed chemically unchanged into the systemic blood circulation. **

The first thing to note is that bioavailability is reported as a proportion. So, if a quarter of the administered dose eventually gets into the circulation, its bioavailability could be quoted as 0.25 or 25%. Either value is perfectly acceptable, but notice that in any pharmacokinetic calculation where a value for bioavailability has to be incorporated, it must be in the decimal form (0.25, not 25).

The second point about the definition is the inclusion of the term ‘chemically unchanged’. This would be especially important with an oral dose of drug, where part of the dose may be chemically degraded during the absorption process and thus we absorb a mixture of authentic, active drug and breakdown products. Only the unchanged material is considered as bioavailable.

Finally, the definition requires the drug to reach the ‘systemic blood circulation: What this means is the general or well-mixed circulation, not some obscure backwater. The requirement for this term will be more obvious, once we have considered oral bioavailability in more detail (next section).

Bioavailability is sometimes called ‘Fractional availability’ and consequently it is represented in pharmacokinetic calculations as ‘F’.

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- Explain Passive Diffusion by Which Drugs May Cross Biological Membranes