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July 18, 1977

Effect of Urinary pH on Renal Excretion of Drugs

JAMA. 1977;238(3):210. doi:10.1001/jama.1977.03280030018002

To the Editor.—  Replying to a question from a reader from Alabama, Azarnoff (237:1151, 1977) stated that the acidification of the urine is used to accelerate the excretion of amphetamine in patients with overdosage.The excretion of the unchanged form of a drug through the urine may depend on the urinary pH only if the pKa of the drug differs from the pH by about two units. This is because the degree of ionization of the drug (and therefore the extent of reabsorption of the drug) is affected only negligibly when the difference between the pH of the fluid and pKa of the drug exceeds two units, as shown in the Table.Since the pH of the urine ranges approximately from 4.8 to 7.5, the urinary excretion of a large number of drugs may be urinary pH-dependent. The following equations may be used to calculate the percentage of ionization of