Cimetidine in the Presence of Renal Impairment

To the Editor.—  Mogelnicki et al (241:826, 1979) describe physostigmine reversal of cimetidine-induced mental confusion in two critically ill patients with impaired renal function. The patients were receiving cimetidine therapy, 300 mg intravenously, every six hours for several days, one for prophylaxis of gastrointestinal hemorrhage and the other, for a gastric ulcer. These cases raise two important considerations in using cimetidine, which may have prevented the obtundation. First, the major route of cimetidine excretion is the kidney.¹ The frequency of its administration is reduced in patients with moderate and severe renal impairment to every eight and 12 hours, respectively. In case 1, progressive azotemia and obtundation developed, giving consideration for reducing the dose frequency. Second, hemodialysis reduces the circulating blood level of cimetidine, usually necessitating an extra 300-mg dose at the end of dialysis.^2,3 In case 2, dialysis probably accounted for the low cimetidine blood level. The dose