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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 folic acid synthesis.
## Primary Indications
* Urinary tract infections (UTIs)
* *Pneumocystis jirovecii* pneumonia (PJP) - treatment and prophylaxis
* Skin and soft tissue infections (e.g., MRSA)
* Traveler's diarrhea
* Stenotrophomonas maltophilia infections
## Adult Dosing
* **Acute Bacterial Infection:** 160 mg trimethoprim/800 mg sulfamethoxazole (1 DS tablet) PO 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 mg trimethoprim/800 mg sulfamethoxazole daily or 3 times weekly.
* *Note: Dosing varies significantly by local protocol and indication severity.*
## Pediatric Dosing
* **General Infection (≥2 months):** 8 mg/kg/day (trimethoprim component) divided every 12 hours.
* **PJP Treatment:** 15–20 mg/kg/day (trimethoprim component) divided every 6 hours.
* **PJP Prophylaxis:** 150 mg/m²/day (trimethoprim component) divided every 12 hours (max 320 mg/day).
## Dose Adjustments
* **Renal Impairment:**
* CrCl 15–30 mL/min: Reduce dose by 50%.
* CrCl <15 mL/min: Not recommended.
* **Hepatic Impairment:** No specific adjustment; use with caution.
## Contraindications
* Hypersensitivity to sulfonamides or trimethoprim.
* History of drug-induced immune thrombocytopenia.
* Megaloblastic anemia due to folate deficiency.
* Pregnancy at term (risk of kernicterus).
* Severe renal or hepatic failure (where blood monitoring is not possible).
## Adverse Effects
* **Common:** Nausea, vomiting, skin rash, pruritus.
* **Serious:** Stevens-Johnson syndrome/TEN, severe neutropenia, agranulocytosis, hyperkalemia (dose-dependent), acute kidney injury, C. difficile-associated diarrhea, QTc prolongation.
## Key Drug Interactions
* **Warfarin:** Significant increase in INR; monitor closely.
* **ACE Inhibitors/ARBs/Spironolactone:** Increased risk of severe hyperkalemia.
* **Methotrexate:** Increased risk of bone marrow suppression due to synergistic antifolate effects.
* **Sulfonylureas:** Enhanced hypoglycemic effect.
* **Dofetilide/QTc prolongers:** Increased risk of cardiac arrhythmias.
## Monitoring
* **Renal function:** Serum creatinine/BUN.
* **Electrolytes:** Specifically potassium (risk of hyperkalemia).
* **Hematology:** CBC (monitor for cytopenias, especially on long-term therapy).
* **Clinical:** Signs of skin rash (discontinue immediately if present).
## Clinical Pearls
* **Hydration:** Maintain adequate fluid intake to prevent crystalluria.
* **Folate Deficiency:** Consider baseline deficiency in elderly or malnourished patients; monitor MCV.
* **MRSA:** Co-trimoxazole retains high activity against many community-acquired MRSA strains.
* **Hyperkalemia:** Particularly relevant in patients taking concomitant RAAS inhibitors or those with renal impairment.
***
**Educational Disclaimer:** This information is for educational purposes only. Always consult current institutional guidelines, local antibiograms, and official prescribing information (e.g., package inserts or clinical resources like Lexicomp or UpToDate) before prescribing or administering medication.
