PREGNANCY: Use of sulfonamides may cause bilirubin to be displaced from proteins in the infant's blood. Displacement of bilirubin can lead to jaundice and a dangerous condition called kernicterus in the infant. For this reason, sulfamethoxazole/trimethoprim should not be used near term (late in pregnancy) among women.

NURSING MOTHERS: Sulfamethoxazole/trimethoprim should not be used by nursing mothers because sulfamethoxazole is excreted in milk and can cause kernicterus.

SIDE EFFECTS: Sulfamethoxazole/trimethoprim may cause dizziness, headache, lethargy, diarrhea, anorexia, nausea, vomiting, and rash. Sulfamethoxazole/trimethoprim should be stopped at the first appearance of a skin rash before the rash becomes severe. Serious rashes include Stevens-Johnson syndrome (aching joints and muscles; redness, blistering, and peeling of the skin); toxic epidermal necrolysis (difficulty in swallowing; peeling, redness, loosening, and blistering of the skin).

Sulfamethoxazole/trimethoprim therapy also can cause extensive sunburn, following exposure to sunlight. Patients receiving Sulfamethoxazole/trimethoprim should avoid excessive exposure to sunlight (photosensitivity to drugs) and should wear sunscreen.

Other rare side effects include liver damage, low white blood cell count, low platelet count (thrombocytopenia), and anemia.

Sulfamethoxazole/trimethoprim may form crystals in the urine which may damage the kidney and cause bleeding into the urine. It is important to drink additional liquids during therapy to prevent these side effects.

Reference: FDA Prescribing Information

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