Effect of antacids on salicylate kinetics

The effect of an antacid on the absorption of aspirin can vary depending on certain physical characteristics of the brand of aspirin chosen. In a study carried out to test the effect of a commonly used antacid on the absorption of a brand of aspirin, no differences were seen in the area under the curve (AUC) of plasma concentrations of salicylic acid versus time, Cmax and Ka of salicylic acid without or with antacid. However, the antacid was seen to alter the pH of urine in four of the six subjects investigated. The study, when extended to examine the effect of antacid on steady-state kinetics of salicylic acid, showed that when the Cmin plasma concentrations of salicylic acid were in the region of 15 mg%, the antacid reduced the plasma concentrations and half-life of salicylic acid. It therefore follows that 15 mg% is the minimum plasma concentration at which an antacid that alters the pH of urine brings about a reduction in the steady-state plasma concentrations and an increase in the elimination of salicylic acid from plasma.