Bioavailability

Bioavailability tells you how much of an orally taken drug actually gets into the bloodstream compared with the same drug given directly into a vein. The AUC, or area under the concentration–time curve, measures total body exposure to the drug over time, so comparing oral AUC with IV AUC shows how much exposure is lost because of incomplete absorption and first-pass metabolism. IV administration is treated as the reference because it delivers the full dose straight into systemic circulation, so its AUC represents 100% availability for the same amount administered.

Use this formula when comparing oral and IV formulations of the same drug, or when estimating how efficiently an oral dose reaches systemic circulation. It is especially useful in pharmacokinetics, drug development, and dose conversion problems.

• Using drug dose values instead of AUC values
• Forgetting to multiply by 100 to convert the ratio into a percentage
• Comparing AUCs from different doses without accounting for dose differences