Which ADME component determines how a drug enters the bloodstream?

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Absorption is the step that determines entry into the bloodstream. It’s the process by which a drug moves from its site of administration into the systemic circulation, and it sets how much of the dose actually reaches the blood. Route and formulation matter—oral, subcutaneous, or transdermal routes each have different barriers, and dissolution and membrane permeability influence how efficiently the drug crosses into blood. Bioavailability—the fraction of the administered dose that reaches circulation—emphasizes this point: if absorption is limited or bypassed (as with intravenous administration, which puts the drug directly into the blood), the amount entering the bloodstream changes accordingly. After absorption, distribution, metabolism, and excretion describe what happens to the drug once it’s already in the bloodstream, not the initial entry itself.