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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
* *Pneumocystis jirovecii* pneumonia (PJP) treatment and prophylaxis.
* Urinary tract infections (UTIs).
* Prostatitis.
* Community-acquired MRSA skin and soft tissue infections.
* Traveler's diarrhea.
## Adult Dosing
* **Mild-Moderate Infection:** 160 mg trimethoprim / 800 mg sulfamethoxazole (one "double strength" or DS 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 mg trimethoprim / 800 mg sulfamethoxazole once daily or three times weekly.
## Pediatric Dosing
* **General Infections (>2 months old):** 8–10 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 twice daily (Max: 160 mg trimethoprim/800 mg sulfamethoxazole/day).
## Dose Adjustments
* **Renal Impairment:**
* CrCl 15–30 mL/min: Reduce dose by 50%.
* CrCl <15 mL/min: Not recommended (unless hemodialysis).
* **Hepatic Impairment:** Use with caution; monitor for potential hepatotoxicity.
## Contraindications
* Hypersensitivity to sulfonamides or trimethoprim.
* Documented megaloblastic anemia due to folate deficiency.
* Severe renal or hepatic insufficiency (when monitoring is not possible).
* Pregnancy at term (risk of kernicterus in neonates).
* Infants under 2 months of age.
## Adverse Effects
* **Common:** Nausea, vomiting, rash, photosensitivity.
* **Serious:** Stevens-Johnson syndrome/Toxic Epidermal Necrolysis, agranulocytosis, hyperkalemia (dose-dependent), crystalluria, and acute kidney injury.
## Key Drug Interactions
* **Warfarin:** Significant increase in INR (inhibits CYP2C9); monitor closely.
* **ACE Inhibitors/ARBs/Spironolactone:** Increased risk of severe hyperkalemia.
* **Methotrexate:** Increased risk of myelosuppression/bone marrow toxicity.
* **Sulfonylureas:** Increased risk of hypoglycemia.
## Monitoring
* **Renal Function:** Serum creatinine and electrolytes (potassium), especially in elderly or those on RAAS inhibitors.
* **Hematology:** CBC with differential (monitor for myelosuppression with prolonged therapy).
* **Hydration:** Ensure adequate fluid intake to prevent crystalluria.
## Clinical Pearls
* **Dosing clarification:** Always specify if dosing based on the trimethoprim or the combined tablet strength.
* **Hyperkalemia:** Co-trimoxazole structurally mimics the potassium-sparing diuretic amiloride; monitor serum potassium within 3–5 days of initiation in high-risk patients.
* **PJP Dosing:** High-dose PJP requirements often necessitate adjusting from the standard oral fixed-dose ratio.
* **Local Protocols:** Empirical dosing for MRSA and UTI resistance varies significantly by geographic region; always consult local antibiograms.
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**Educational Disclaimer:** This information is for educational purposes only. Drug dosing, contraindications, and interaction profiles can change. Always verify current prescribing information using local institutional protocols, pharmaceutical package inserts, or clinical decision support tools before prescribing or administering medication.
