Bioavailability is an essential property of medications The amount of a substance that reaches its final destination in the body. Yeah, some of the active substance is "lost" along the way☝🏻 It depends on the method of administration of the drug (by mouth, intramuscularly, subcutaneously, intravenously, etc.), the state of the body and the properties of the drug itself. Like, the bioavailability of the well-known analgin (analgesic) is about 90% when administered by mouth.