Known brand names include Bactrim and Sulfatrim.
A combination antibiotic pill (a sulfa drug plus trimethoprim) for urinary tract infections, ear infections in children, bronchitis flares, traveler's diarrhea, shigellosis, and Pneumocystis pneumonia. Rash is common and severe skin reactions can occur; it can also raise potassium and strengthen warfarin.
Labeled use. Susceptible gram-negative organisms; a single agent is preferred for an initial uncomplicated UTI.
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Labeled use. Treatment and prophylaxis in immunosuppressed patients.
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Labeled use. Acute otitis media in children; not for strep throat.
A first-line option for uncomplicated cystitis where local E. coli resistance is low, the drug of choice to treat and prevent Pneumocystis pneumonia, and a common oral choice for community MRSA skin infections. Avoid with sulfa allergy, in late pregnancy and young infants, and with warfarin, high potassium risk, or significant renal impairment.
Sulfamethoxazole competes with para-aminobenzoic acid (PABA) to block bacterial dihydrofolic acid synthesis, and trimethoprim reversibly inhibits dihydrofolate reductase, blocking the next step to tetrahydrofolic acid. Together they block two consecutive steps in nucleic acid and protein synthesis, and resistance develops more slowly than with either drug alone. Both are cleared mainly by the kidneys, with half-lives of about 10 hours (sulfamethoxazole) and 8 to 10 hours (trimethoprim). Trimethoprim inhibits CYP2C8 and OCT2; sulfamethoxazole inhibits CYP2C9.
The label does not describe a narrow therapeutic index or direct dosing by measured blood levels of the drug.
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Doses come from the FDA label. Study tool, not for clinical decisions.
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Creatinine clearance above 30: usual regimen. 15 to 30: half the usual regimen. Below 15: not recommended. Half-lives of both components rise in severe renal impairment.
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No specific antidote. With normal kidneys and low urine output, give IV fluids; acidifying the urine speeds trimethoprim elimination. Hemodialysis is only moderately effective. For bone marrow depression from high-dose or prolonged use, give leucovorin 5 to 15 mg daily until blood counts recover.
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Some studies link first-trimester exposure to birth defects, including neural tube, heart, and urinary tract defects, oral clefts, and club foot, while others do not. Because it interferes with folate metabolism, use only if the benefit justifies the risk, and tell the patient about the potential hazards.
Milk levels are about 2% to 5% of the infant dose. Use caution, especially with jaundiced, ill, stressed, or premature infants, because of bilirubin displacement and kernicterus risk.
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Contraindicated under 2 months. UTI or otitis media: 8 mg/kg trimethoprim and 40 mg/kg sulfamethoxazole per day in two doses for 10 days. Not for prophylactic or prolonged use in otitis media, and data on repeated use under age 2 are limited.
Higher risk of severe reactions, especially severe skin reactions, bone marrow suppression, low platelets, and hyperkalemia. Thiazides add to thrombocytopenia, and digoxin levels can rise. Adjust for renal function and keep courses short.
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