Absorption is best described as:

• A. Movement of drug into circulation
• B. Release of the drug from the dosage form
• C. Distribution to tissues
• D. Metabolism by liver enzymes

Answer: (A)

Absorption is the movement of a drug from its site of administration into the bloodstream, making it available for distribution to tissues and sites of action. This step determines how much of the dose actually reaches systemic circulation (bioavailability).

This is distinct from release from the dosage form, which refers to the drug disintegrating and dissolving out of a tablet or capsule before it can be absorbed. It is also different from distribution to tissues, which is the drug moving from blood into organs and other compartments after it has entered circulation. And it is not metabolism by liver enzymes, which occurs after the drug is in the body and often alters its chemical form.

For oral drugs, absorption depends on factors like solubility, ionization (which is pH-dependent), lipid solubility, surface area of the absorbing site (e.g., small intestine), and first-pass metabolism in the liver that can reduce the amount reaching systemic circulation.