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# Septran (Co-trimoxazole: Sulfamethoxazole/Trimethoprim)
## Overview
Co-trimoxazole is a fixed-dose combination antibiotic (5:1 ratio of sulfamethoxazole to trimethoprim). It acts as a synergistic inhibitor of bacterial folate synthesis. Dosing is traditionally expressed by the sulfamethoxazole (SMX) component.
## Primary Indications
* Urinary tract infections (UTI).
* *Pneumocystis jirovecii* pneumonia (PJP) treatment and prophylaxis.
* Methicillin-resistant *Staphylococcus aureus* (MRSA) skin and soft tissue infections.
* Traveler's diarrhea (shigellosis).
## Adult Dosing
* **UTI (Uncomplicated):** 800/160 mg PO every 12 hours for 3 days.
* **Severe Infections:** 8–10 mg/kg/day (based on trimethoprim component) IV/PO in divided doses every 6, 8, or 12 hours.
* **PJP Treatment:** 15–20 mg/kg/day (based on trimethoprim) IV/PO in 3–4 divided doses for 14–21 days.
* **PJP Prophylaxis:** 800/160 mg PO once daily or three times weekly.
## Pediatric Dosing
* **General Infections (>2 months old):** 8–10 mg/kg/day (trimethoprim component) divided every 12 hours.
* **PJP Treatment:** 15–20 mg/kg/day (trimethoprim component) divided every 6–8 hours.
* **PJP Prophylaxis:** 150 mg/m²/day (trimethoprim) divided twice daily (3 days/week).
## Dose Adjustments
* **Renal Impairment:**
* CrCl 15–30 mL/min: Reduce dose by 50%.
* CrCl <15 mL/min: Not recommended unless hemodialysis is available.
* **Hepatic Impairment:** Use with caution; monitor for hepatotoxicity.
## Contraindications
* Hypersensitivity to sulfonamides or trimethoprim.
* Documented megaloblastic anemia due to folate deficiency.
* Pregnancy at term (risk of kernicterus in neonate).
* Severe renal or hepatic failure (where monitoring isn't possible).
* History of drug-induced immune thrombocytopenia.
## Adverse Effects
* **Common:** Nausea, vomiting, diarrhea, rashes (common in HIV-positive patients).
* **Serious:** Stevens-Johnson syndrome/TEN, hyperkalemia (dose-dependent), acute kidney injury, bone marrow suppression (leukopenia, agranulocytosis), and *Clostridioides difficile*-associated diarrhea.
## Key Drug Interactions
* **Warfarin:** Significant increase in INR; monitor closely and consider dose reduction.
* **ACE Inhibitors/ARBs/Spironolactone:** High risk of hyperkalemia.
* **Methotrexate:** Increased risk of pancytopenia due to additive folate antagonism.
* **Dofetilide:** Contraindicated; risk of increased serum concentration and QT prolongation.
## Monitoring
* **Renal Function:** Serum creatinine and electrolytes (especially potassium).
* **Hematology:** CBC (monitor for baseline cytopenias and during prolonged therapy).
* **Clinical:** Skin assessment for rashes; monitor for signs of hypersensitivity.
## Clinical Pearls
* **Hydration:** Always instruct patients to drink plenty of fluids to prevent crystalluria.
* **Dosing Confusion:** Ensure the distinction between SS (Single Strength: 400/80 mg) and DS (Double Strength: 800/160 mg) is clearly communicated.
* **Local Protocols:** Resistance patterns vary significantly geographically; always consult local antibiograms before empirical use in complicated infections.
* **Folate:** High-dose, long-term therapy may require prophylactic or therapeutic folate supplementation to mitigate hematologic toxicity.
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**Educational Disclaimer:** This information is for educational purposes only. Dosing, contraindications, and interactions must be verified against current institutional guidelines, local resistance patterns, and the most recent prescribing information (e.g., FDA label or BNF) before clinical decision-making.
