Absorption is best defined as which of the following?

Absorption refers specifically to the process by which a drug is taken up from the site of administration and enters the bloodstream. This is a critical phase in pharmacokinetics, as it determines the onset of action and the overall bioavailability of the medication. The effectiveness of a drug depends significantly on how well it is absorbed; this can be influenced by various factors such as the route of administration, the formulation of the drug, and the physiological condition of the individual.

In contrast, the other options describe different pharmacokinetic processes. The first option addresses drug metabolism, which involves the transformation of drugs into more or less active forms, but does not pertain to the absorption phase. The third option discusses drug distribution, which is the movement of drugs through the bloodstream to various target tissues after they have been absorbed. The fourth option pertains to elimination or excretion, which involves the removal of drugs from the body, typically through urine or feces. Each of these processes occurs at different stages in the life cycle of a drug within the body, making the definition of absorption uniquely focused on its own specific process.