Hyperkalemia and trimethoprim-sulfamethoxazole: a new problem emerges 25 years later.

Trimethoprim-sulfamethoxazole is a frequently prescribed antibiotic with a wide spectrum of antimicrobial activity. A previously unreported and potentially lethal adverse reaction associated with "high dose" trimethoprim-sulfamethoxazole therapy, hyperkalemia, was described. Subsequent to the descriptions of hyperkalemia with "high dose" trimethoprim-sulfamethoxazole, a handful of cases noted the development of hyperkalemia with "standard dose" trimethoprim-sulfamethoxazole in elderly patients without evidence of an obvious defect in potassium homeostasis. A surveillance study of patients treated with "standard dose" trimethoprim-sulfamethoxazole as compared to similar controls treated with other antibiotics confirmed the rise in potassium associated with trimethoprim-sulfamethoxazole therapy. Recognition of this potassium disorder led to investigation and description of the mechanism by which trimethoprim-sulfamethoxazole-induced hyperkalemia. Trimethoprim was found to act like the potassium-sparing diuretic amiloride and reduce renal potassium excretion. Hence, trimethoprim-sulfamethoxazole therapy was found to be associated with a new adverse reaction, hyperkalemia, nearly 25 years after its introduction into clinical practice as an antimicrobial agent.