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# Septran (co-Trimoxazole / Trimethoprim-Sulfamethoxazole)
## Overview
Co-trimoxazole is a synergistic combination of a sulfonamide (sulfamethoxazole) and a dihydrofolate reductase inhibitor (trimethoprim) in a fixed 5:1 ratio. It is bactericidal against a wide range of Gram-positive and Gram-negative organisms.
## Primary Indications
* Urinary tract infections (UTI)
* *Pneumocystis jirovecii* pneumonia (PJP) treatment and prophylaxis
* Community-acquired MRSA (skin/soft tissue infections)
* Traveler’s diarrhea
* Stenotrophomonas maltophilia infections
## Adult Dosing
* **Acute Bacterial Infections:** 160 mg trimethoprim / 800 mg sulfamethoxazole (one double-strength tablet) every 12 hours.
* **PJP Treatment:** 15–20 mg/kg/day (based on trimethoprim component) divided every 6–8 hours for 21 days.
* **PJP Prophylaxis:** 160/800 mg once daily or 80/400 mg once daily (depends on patient risk and local protocol).
## Pediatric Dosing (>2 months)
* **Acute Bacterial Infections:** 8 mg/kg/day (based on trimethoprim) divided every 12 hours.
* **PJP Treatment:** 15–20 mg/kg/day (based on trimethoprim) divided every 6–8 hours.
* **PJP Prophylaxis:** 150 mg/m²/day (based on trimethoprim) divided into two doses twice daily, 3 days per week (consecutive).
## Dose Adjustments
* **Renal Impairment:** Adjust based on CrCl. If CrCl 15–30 mL/min, reduce dose by 50%. Administering if CrCl <15 mL/min is generally not recommended.
* **Hepatic Impairment:** Use with caution; monitor closely for jaundice or hepatotoxicity.
## Contraindications
* Known hypersensitivity to sulfonamides or trimethoprim.
* Documented megaloblastic anemia due to folate deficiency.
* Severe renal or hepatic impairment (where levels cannot be monitored).
* Pregnancy at term (risk of kernicterus in neonate).
* Infants <2 months of age (risk of kernicterus).
## Adverse Effects
* **Common:** Rash, nausea, vomiting, photosensitivity.
* **Serious:** Stevens-Johnson syndrome (SJS), Toxic Epidermal Necrolysis (TEN), agranulocytosis, hyperkalemia (dose-dependent), crystalluria, and acute kidney injury (interstitial nephritis).
## Key Drug Interactions
* **Warfarin:** Significant increase in INR; monitor closely and consider empiric dose reduction of warfarin.
* **ACE Inhibitors/ARBs/Potassium-Sparing Diuretics:** Increased risk of severe hyperkalemia.
* **Methotrexate:** Increased risk of bone marrow suppression due to synergistic antifolate effect.
* **Sulfonylureas:** Increased risk of hypoglycemia.
## Monitoring
* **Renal Function:** Monitor serum creatinine and BUN.
* **Electrolytes:** Monitor serum potassium, especially in patients on RAAS inhibitors or with baseline renal impairment.
* **Hematology:** CBC with differential (long-term use can cause bone marrow suppression).
* **Clinical:** Skin integrity (watch for SJS/TEN signs).
## Clinical Pearls
* **Hydration:** Maintain adequate fluid intake to prevent crystalluria.
* **Hyperkalemia:** Even standard doses can cause clinically significant hyperkalemia, particularly in the elderly.
* **Local Resistance:** Sensitivity patterns vary highly by geography; always consult local antibiograms.
* **Folate:** Supplementation with folinic acid (leucovorin) may be considered during high-dose PJP treatment to reduce myelosuppression, but avoid folic acid as it may interfere with antibiotic efficacy.
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*Disclaimer: This information is for educational purposes only. Clinical practice guidelines and local resistance patterns vary. Always verify current prescribing information, institutional protocols, and patient-specific factors via official resources (e.g., Lexicomp, local formulary) before prescribing or administering medication.*
