Acute mental confusion occurred in a patient on a stable dose of amantadine after he was started on trimethoprim-sulfamethoxazole. Discontinuation of the drugs led to rapid improvement. Studies in a rat model of organic cation secretion demonstrate that both amantadine and trimethoprim, but not sulfamethoxazole, inhibit the renal secretion of the organic cation procainamide. This case may represent an amantadine-trimethoprim interaction.
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