The effect of spontaneous changes in urinary pH on mexiletine plasma concentrations and excretion during chronic administration to healthy volunteers.

1. The effect of spontaneous changes in urinary pH on renal excretion and plasma concentration of mexiletine has been examined during chronic administration of subtherapeutic doses of the drug to healthy volunteers. 2. Significant correlations were found between urinary pH and the plasma concentration and renal excretion of mexiletine. 3. Prediction of plasma mexiletine concentrations from our data suggests that the amount of mexiletine in plasma would increase by more than 50% following a rise in urinary pH similar to that which occurred spontaneously in our subjects. 4. Factors which influence urinary pH should be considered when the dosage of mexiletine is chosen. Extremes of urinary pH may account for some cases of inefficacy of the drug, and for the occurrence of unwanted effects at conventional doses.