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# Septran (co-Trimoxazole)
## Overview
Co-trimoxazole consists of sulfamethoxazole (SMX) and trimethoprim (TMP) in a fixed 5:1 ratio. It acts by inhibiting sequential steps in bacterial folic acid synthesis.
## Primary Indications
* **Respiratory:** *Pneumocystis jirovecii* pneumonia (PJP) treatment and prophylaxis, acute exacerbations of chronic bronchitis.
* **Urinary:** Uncomplicated urinary tract infections (UTIs).
* **GI:** Traveler's diarrhea, shigellosis.
* **Skin/Soft Tissue:** MRSA-susceptible skin and soft tissue infections.
* **Systemic:** Stenotrophomonas maltophilia infections, nocardiosis.
## Adult Dosing
* **Uncomplicated UTI:** 160/800 mg (one double-strength tablet) every 12 hours for 3 days.
* **PJP Treatment:** 15–20 mg/kg/day (based on TMP component) divided every 6–8 hours for 14–21 days.
* **PJP Prophylaxis:** 160/800 mg once daily or three times weekly.
* **Severe Systemic Infection:** 8–10 mg/kg/day (based on TMP) divided every 12 hours.
## Pediatric Dosing
* **General infection (≥2 months old):** 8–10 mg/kg/day (based on TMP) divided every 12 hours.
* **PJP Treatment:** 15–20 mg/kg/day (based on TMP) divided every 6–8 hours.
* **PJP Prophylaxis:** 150 mg/m²/day (based on TMP) divided every 12 hours (three times weekly).
* *Note: Maximum doses usually follow adult limits unless weight-based dosing dictates otherwise.*
## Dose Adjustments
* **Renal Impairment:**
* CrCl 15–30 mL/min: Reduce dose by 50%.
* CrCl <15 mL/min: Not recommended.
* **Hepatic Impairment:** Use with caution; monitor for hyperkalemia and 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 neonates).
* Infants <2 months old (risk of kernicterus).
## Adverse Effects
* **Common:** Nausea, vomiting, rash, pruritus.
* **Serious:** Stevens-Johnson syndrome (SJS)/Toxic Epidermal Necrolysis (TEN), hyperkalemia, agranulocytosis, crystalluria, acute kidney injury (interstitial nephritis).
## Key Drug Interactions
* **Warfarin:** Significant potentiation of anticoagulant effect (monitor INR closely).
* **ACE Inhibitors/ARBs/Spironolactone:** Increased risk of severe hyperkalemia.
* **Methotrexate:** Increased risk of marrow suppression due to synergistic folate antagonism.
* **Dofetilide:** Contraindicated; co-trimoxazole increases dofetilide plasma concentrations, risking Torsades de Pointes.
## Monitoring
* **Electrolytes:** Serum potassium (especially in renal impairment, elderly, or concurrent RAAS inhibitors).
* **Renal Function:** Serum creatinine/BUN at baseline and during prolonged therapy.
* **Hematology:** CBC (monitor for cytopenias during long-term prophylaxis).
## Clinical Pearls
* **Hydration:** Maintain adequate fluid intake to prevent crystalluria.
* **Hyperkalemia:** TMP acts similarly to amiloride (potassium-sparing effect) on the renal distal tubule.
* **PJP Dosing:** Always specify if dosing is based on the TMP component for clarity, as clinicians may confuse it with the SMX component.
* **Local Protocols:** Always verify local susceptibilities; resistance patterns for *E. coli* are high in many regions.
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*Disclaimer: This information is for educational purposes and does not substitute for clinical judgment. Always verify current prescribing information, institutional guidelines, and patient-specific factors before administration.*
