In pharmacology, bioavailability's used to describe the fraction of an administered dose of unchanged drug that reaches the systemic circulation, one of the principal pharmacokinetic properties of drugs. By definition, when a medication's administered intravenously, its bioavailability's 100%. However, when a medication's administered via other routes (such as orally), its bioavailability decreases (due to incomplete absorption and first-pass metabolism). Bioavailability's one of the essential t… (More on Bioavailability)