In medicine, bioavailability may be a subcategory of absorption associated is that the fraction of an administered drug that reaches the circulation. By definition, once a drugs is run intravenously, its bioavailability is 100%. However, once a drugs is run via routes aside from endovenous, its bioavailability is mostly below that of endovenous thanks to internal organ epithelial tissue absorption and first-pass metabolism. Thereby, mathematically, bioavailability equals the magnitude relation of comparison the world underneath the plasma drug concentration curve versus time (AUC) for the extravascular formulation to the AUC for the intravascular formulation.