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# Septran (Co-trimoxazole)
## Overview
Co-trimoxazole is a fixed-dose combination of sulfamethoxazole (a sulfonamide) and trimethoprim (a folate antagonist) in a 5:1 ratio. It acts synergistically to inhibit sequential steps in bacterial folic acid synthesis.
## Primary Indications
* Urinary tract infections (UTIs).
* *Pneumocystis jirovecii* pneumonia (PJP) treatment and prophylaxis.
* Methicillin-resistant *Staphylococcus aureus* (MRSA) skin and soft tissue infections.
* Traveler’s diarrhea and *Stenotrophomonas maltophilia* infections.
## Adult Dosing
* **UTI:** 160mg/800mg (DS tablet) every 12 hours for 3 days.
* **PJP Treatment:** 15–20 mg/kg/day (based on trimethoprim component) divided in 3–4 doses.
* **PJP Prophylaxis:** 160mg/800mg once daily or three times weekly.
* **Skin/Soft Tissue:** 160mg/800mg every 12 hours.
## Pediatric Dosing
* **General Infections:** 8 mg/kg/day (trimethoprim component) divided every 12 hours.
* **PJP Treatment:** 15–20 mg/kg/day (trimethoprim component) divided every 6–8 hours.
* **Note:** Not recommended for infants < 2 months of age due to risk of kernicterus.
## Dose Adjustments
* **Renal Impairment:** If CrCl 15–30 mL/min, reduce dose by 50%. Avoid if CrCl < 15 mL/min.
* **Hepatic Impairment:** Use with caution; monitor liver function closely.
## Contraindications
* Hypersensitivity to sulfonamides or trimethoprim.
* Documented megaloblastic anemia due to folate deficiency.
* Severe renal or hepatic impairment.
* Pregnancy at term (risk of kernicterus in neonate).
* History of drug-induced immune thrombocytopenia.
## Adverse Effects
* **Common:** Nausea, vomiting, skin rash, pruritus.
* **Serious:** Stevens-Johnson syndrome (SJS), toxic epidermal necrolysis (TEN), hyperkalemia (especially in high doses or with ACE inhibitors/ARBs), bone marrow suppression (leukopenia/thrombocytopenia), and acute kidney injury (AIN).
## Key Drug Interactions
* **Warfarin:** Significantly increases INR; requires close monitoring and dose reduction of warfarin.
* **Potassium-sparing diuretics/ACE inhibitors/ARBs:** Increased risk of severe hyperkalemia.
* **Methotrexate:** Increased risk of toxicity due to folate antagonism.
* **Sulfonylureas:** Potential potentiation of hypoglycemic effects.
## Monitoring
* **Renal Function:** Monitor serum creatinine and BUN.
* **Electrolytes:** Serum potassium (especially in renal patients or those on interacting drugs).
* **Hematology:** CBC (baseline and periodically during long-term therapy) to monitor for blood dyscrasias.
## Clinical Pearls
* **Hydration:** Maintain adequate fluid intake to prevent crystalluria.
* **Microbiology:** Resistance patterns vary significantly by geography; always consult local institutional antibiograms.
* **PJP Dosing:** Always denote whether the dose is based on the trimethoprim or the combined components to avoid dosing errors.
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*Disclaimer: This information is for educational purposes and does not replace professional clinical judgment. Always consult current institutional prescribing guidelines, drug monographs, and the most recent package insert before prescribing or administering medication.*
