Pharmacokinetics and Pharmacodynamics

Pharmacokinetics

Aspirin is a weak organic acid, with a pKa of 3.5. Taken orally, it is absorbed in the stomach and the upper small intestine, mainly in the non-ionized form due to the acidic conditions in the stomach. Once absorbed, it is metabolized to acetic acid and salicylate by esterases in tissue and blood (see Figure 4). The free salicylate is then excreted unchanged, or converted to other water-soluble compounds that are then excreted by the kidney.

Metabolism 

of Aspirin
Figure 4.

The excretion of salicylate occurs with first-order kinetics with a half-life between 2 and 19 hours, depending on the dose of aspirin administered.

Pharmacodynamics

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Author: Kathryn Will
(Document Modification Date: 17-May-2000)

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