Which ADME component determines how quickly a drug enters systemic circulation?

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The rate at which a drug appears in the bloodstream is determined by absorption. Absorption covers how the drug moves from the site of administration into the systemic circulation, and it is governed by factors like solubility and dissolution, the drug’s ability to cross membranes, the route taken, surface area at the absorption site, and local blood flow. For oral medications, the time it takes to dissolve in the GI tract and cross the intestinal wall largely sets how quickly the drug becomes available in the blood, influencing the onset of action. After the drug has entered the bloodstream, distribution decides how it spreads to tissues, metabolism alters how quickly it is chemically transformed, and excretion removes it from the body. While metabolism (including first-pass effects) can affect how much of the dose eventually reaches systemic circulation, it does not control the initial entry rate; that is the job of absorption.