Drug interactions - Excretion

Excretion interactions primarily involve changes in renal excretion. This might be due to drug-induced reduction in glomerular filtration rate (e.g. diuretic-induced dehydration, ACE inhibitors, NSAIDs). This can reduce the clearance and increase the plasma concentration of many drugs, including some with a low therapeutic index (e.g. digoxin, lithium, aminoglycoside antibiotics). Less commonly, interactions may be due to competition for a common tubular organic anion transporters (e.g. methotrexate excretion may be inhibited by competition with NSAIDs). In some cases this kind of interaction can be used for therapeutic benefit (e.g. probenecid prolongs the half life of penicillin be competing for tubular excretion). The likelihood of clinically significant excretion interactions increases if a patient has renal impairment.