Clinically important interactions, both beneficial and potentially harmful, occur in the kidney.
Interference with passive diffusion (see p. 96). Reabsorption of a drug by the renal tubule can be reduced, and its excretion increased, by altering urine pH (see Drug overdose, p. 155).
Interference with active transport. Organic acids are passed from the blood into the urine by active transport across the renal tubular epithelium. Penicillin is mostly excreted in this way. Probenecid, an organic acid that competes successfully with penicillin for this transport system, may be used to prolong the action of penicillin when repeated administration is impracticable, e.g. in sexually transmitted diseases, where compliance is notoriously poor. Interference with renal excretion of methotrexate by aspirin, of zidovudine by probenecid and of digoxin by quinidine, contribute to the potentially harmful interactions with these combinations.
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