Acute

The amount of a single dose of sulfamethoxazole that is either associated with symptoms of overdosage or is likely to be life-threatening has not been reported. Signs and symptoms of overdosage reported with sulfonamides include anorexia, colic, nausea, vomiting, dizziness, headache, drowsiness and unconsciousness. Pyrexia, hematuria and crystalluria may be noted. Blood dyscrasias and jaundice are potential late manifestations of overdosage.

General principles of treatment include the institution of gastric lavage or emesis; forcing oral fluids; and the administration of intravenous fluids if urine output is low and renal function is normal. The patient should be monitored with blood counts and appropriate blood chemistries, including electrolytes. If a significant blood dyscrasia or jaundice occurs, specific therapy should be instituted for these complications. Peritoneal dialysis is not effective and hemodialysis is only moderately effective in eliminating sulfamethoxazole.

Chronic

Use of sulfamethoxazole at high doses and/or for extended periods of time may cause bone marrow depression manifested as thrombocytopenia, leukopenia and/or megaloblastic anemia. If signs of bone marrow depression occur, the patient should be given leucovorin 3 mg to 6 mg intramuscularly daily for 3 days, or as required to restore normal hematopoiesis.

Animal Toxicity

The oral LD₅₀ of sulfamethoxazole is 2300 mg/kg in mice, 3000 mg/kg in rats and > 2000 mg/kg in rabbits.

Hypersensitivity to sulfonamides. Pediatric patients less than 2 months of age (except in the treatment of congenital toxoplasmosis as adjunctive therapy with pyrimethamine). Pregnancy at term and during the nursing period because sulfonamides pass the placenta and are excreted in the milk and may cause kernicterus.

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