Transportation of the unmetabolized drug from the site of administration to the blood circulation.

Absorption depends on a drug’s properties (lipid solubility, molecule size, degree of ionization), route of administration (oral, intramuscular, inhaled, topical, buccal, sublingual, subcutaneous, rectal), formulation (capsules, tablets, liquid, sustained release, lyophilized, cream, gel), patient factors (age, comorbidities, sex, genetic background).
Absorption is one of the key factors affecting the concentration and distribution of a drug within the body and the time it spends in the body (see ADME).