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# Septran (Co-trimoxazole)
## Overview
Co-trimoxazole is a fixed-dose combination of sulfamethoxazole (a sulfonamide) and trimethoprim (a dihydrofolate reductase inhibitor) in a 5:1 ratio. It acts synergistically to inhibit sequential steps in bacterial folate synthesis.
## Primary Indications
* **Urinary Tract Infections (UTI):** Acute uncomplicated cystitis.
* **Respiratory Infections:** Exacerbations of chronic bronchitis, *Pneumocystis jirovecii* pneumonia (PJP) treatment and prophylaxis.
* **Gastrointestinal Infections:** Shigellosis, traveler’s diarrhea.
* **Skin and Soft Tissue Infections:** Susceptible MRSA infections.
## Adult Dosing
* **Uncomplicated UTI:** 160mg/800mg (DS tablet) every 12 hours for 3 days.
* **PJP Treatment:** 15–20 mg/kg/day (based on trimethoprim component) divided every 6–8 hours for 14–21 days.
* **PJP Prophylaxis:** 160mg/800mg once daily or thrice weekly.
## Pediatric Dosing (Weight-based)
* **General Infection:** 8 mg/kg/day of trimethoprim component, divided every 12 hours.
* **PJP Treatment:** 15–20 mg/kg/day of trimethoprim component, divided every 6–8 hours.
* **PJP Prophylaxis:** 150 mg/m²/day of trimethoprim, divided every 12 hours, 3 days a week.
* *Note: Not for infants <2 months due to risk of kernicterus.*
## Dose Adjustments
* **Renal Impairment (CrCl):**
* 15–30 mL/min: Reduce dose by 50%.
* <15 mL/min: Not recommended.
* **Hepatic Impairment:** Use with caution; monitor for hepatotoxicity.
## Contraindications
* Hypersensitivity to sulfonamides or trimethoprim.
* Documented megaloblastic anemia due to folate deficiency.
* Severe renal or hepatic insufficiency.
* Pregnancy at term (risk of kernicterus in neonate).
* Infants <2 months of age.
## Adverse Effects
* **Common:** Nausea, vomiting, diarrhea, anorexia, skin rash (including SJS/TEN).
* **Serious:** Bone marrow suppression (agranulocytosis, thrombocytopenia), hyperkalemia, crystalluria (ensure adequate hydration), nephrotoxicity, and *C. difficile*-associated diarrhea.
## Key Drug Interactions
* **Warfarin:** Significant increase in INR; monitor closely.
* **ACE Inhibitors/ARBs/Spironolactone:** Synergistic risk of severe hyperkalemia.
* **Methotrexate:** Increased risk of myelosuppression due to additive antifolate effect.
* **Phenytoin:** Inhibits metabolism, potentially increasing phenytoin levels.
## Monitoring
* **Renal/Electrolytes:** Baseline and periodic serum creatinine, BUN, and potassium (especially in elderly or patients on RAAS-inhibiting drugs).
* **Hematology:** CBC (especially during prolonged therapy) for signs of bone marrow suppression.
* **Hydration:** Ensure adequate fluid intake to prevent crystalluria.
## Clinical Pearls
* **Dosing Confusion:** Always specify dose based on the trimethoprim component or the combination ratio (i.e., 80/400mg SS vs 160/800mg DS).
* **Hyperkalemia:** Trimethoprim acts mechanistically like amiloride (a potassium-sparing diuretic) at the distal renal tubule.
* **Sulfa Allergy:** Cross-reactivity with other sulfonamides (e.g., sulfonylureas, furosemide) is often debated, but clinically significant immune-mediated reactions warrant caution.
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**Educational Disclaimer:** This information is for educational purposes only. Dosing, contraindications, and drug interactions must be verified against current institutional protocols, local susceptibility patterns, and up-to-date prescribing information (e.g., package inserts or clinical resources like Lexicomp/UpToDate) prior to prescribing.
