What does the term ‘bioavailability’ refer to?

Bioavailability refers to the fraction of an administered dose of unchanged drug that reaches systemic circulation, making it a crucial concept in pharmacokinetics. This term is essential for understanding how much of the drug is available for the body to use after it has been administered, whether orally or via other routes.

When a drug is taken, it must be absorbed into the bloodstream to exert its therapeutic effects. Not all of the drug that is taken will make it to systemic circulation—some may be metabolized, excreted, or lost during the absorption process. Therefore, bioavailability provides a measure of the efficacy of a particular drug formulation and administration route in delivering the active ingredient to the site of action.

This definition is vital for comparing different formulations, such as oral versus intravenous administrations, as intravenous injections typically have 100% bioavailability, while oral drugs may have variable bioavailability due to factors like first-pass metabolism. Understanding bioavailability helps in dose calculations, ensuring that patients receive an effective amount of medication for therapeutic purposes.