Bioavailability is a term used in pharmacology to describe the extent and rate at which a drug or its active component enters systemic circulation, thereby accessing the site of action. It is the fraction of the initial dose of a drug that successfully reaches either the site of action or the bodily fluid domain from which the drugs intended targets have unimpeded access. Bioavailability is an integral part of the pharmacokinetics paradigm, which is the study of drug movement through the body.

Bioavailability is usually assessed by determining the area under the plasma concentration-time curve (AUC), which is directly proportional to the total amount of unchanged drug that reaches systemic circulation. The most reliable measure of a drug’s bioavailability is AUC. Causes of low bioavailability include poor absorption, first-pass metabolism, and drug interactions.

It is important to note that bioavailability is not always 100%, and it can vary depending on the route of administration and other factors. For example, intravenously administered active drugs delivered directly into systemic circulation yield a bioavailability of 100% . Bioavailability is measured by calculating the AUC of the drug concentration-time profile, and it is denoted by the letter f (or, if expressed in percent, by F) .

In summary, bioavailability is a measure of the rate and fraction of the initial dose of a drug that successfully reaches either the site of action or the bodily fluid domain from which the drugs intended targets have unimpeded access. It is an important concept in pharmacology and is used to assess the effectiveness of drugs.