Drug absorption

Drug absorption is the movement of a drug from its administration site into the bloodstream. Its rate and extent depend on the route, drug properties, and local physiology.

Pharmacokinetics: The study of how the body absorbs, distributes, metabolizes, and excretes drugs. Absorption is the entry phase in the processes pharmacokinetics describes.
Oral administration: Delivery of a drug by swallowing it for absorption through the gastrointestinal tract. It exposes drugs to gastrointestinal dissolution, gut-wall transport, and first-pass metabolism.
pH partition hypothesis: A model predicting that weak acids and bases cross membranes according to ionization and pH gradients. It links drug ionization in gastrointestinal fluids to membrane passage.
Drug distribution: The reversible movement of a drug between blood and body tissues. Distribution begins after entry into systemic circulation, unlike absorption from the administration site.
Bioavailability: The fraction of an administered drug dose that reaches systemic circulation unchanged. Absorption contributes to bioavailability, alongside losses such as first-pass metabolism.
Sublingual administration: Placement of a drug beneath the tongue for absorption through the oral mucosa. The oral mucosa can deliver some drugs into circulation while bypassing much first-pass metabolism.
Lipophilicity: The tendency of a molecule to dissolve in lipids rather than water. Membrane partitioning often depends on a drug's balance of lipid and water solubility.
Drug metabolism: The chemical alteration of drugs by the body, often through enzymes. Metabolism can occur before or after systemic entry, but is distinct from absorption itself.
Passive diffusion: Movement of molecules across a membrane down a concentration gradient without energy input. Many drugs cross biological membranes by diffusing down concentration gradients.
Transdermal drug delivery: Delivery of a drug across the skin into systemic circulation. Skin absorption enables sustained systemic delivery without gastrointestinal uptake.

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