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# Septran (co-Trimoxazole)
## Overview
Co-trimoxazole is a fixed-dose combination antibiotic containing Sulfamethoxazole (SMX) and Trimethoprim (TMP) in a 5:1 ratio. It acts via sequential inhibition of folate synthesis.
## Primary Indications
* Urinary tract infections (UTIs)
* *Pneumocystis jirovecii* pneumonia (PJP/PCP) treatment and prophylaxis
* Community-acquired MRSA skin and soft tissue infections
* Traveler’s diarrhea
* Stenotrophomonas maltophilia infections
## Adult Dosing
* **Mild/Moderate Infection:** 1 double-strength (DS) tablet (800 mg SMX/160 mg TMP) twice daily.
* **PJP Treatment:** 15–20 mg/kg/day (based on TMP component) divided every 6–8 hours.
* **PJP Prophylaxis:** 1 single-strength (SS) tablet (400 mg SMX/80 mg TMP) daily or 1 DS tablet 3 times weekly.
## Pediatric Dosing
* **General Infection:** 8 mg/kg/day TMP component orally in two divided doses every 12 hours.
* **PJP Treatment:** 15–20 mg/kg/day (based on TMP) divided every 6–8 hours.
* **PJP Prophylaxis:** 150 mg/m²/day TMP component divided twice daily (3 days/week).
* *Note: Not for use in neonates <2 months due to risk of kernicterus.*
## 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 closely.
## Contraindications
* Hypersensitivity to sulfonamides or trimethoprim.
* Documented megaloblastic anemia due to folate deficiency.
* Severe renal or hepatic failure.
* Pregnancy at term (risk of kernicterus in the neonate).
## Adverse Effects
* **Common:** Rash, nausea, vomiting, pruritus.
* **Serious:** Stevens-Johnson syndrome (SJS), Toxic Epidermal Necrolysis (TEN), acute kidney injury (AKI), hyperkalemia, agranulocytosis, and hepatotoxicity.
## Key Drug Interactions
* **Warfarin:** Significant increase in INR (displaces warfarin and inhibits CYP2C9).
* **ACE Inhibitors/ARBs/Spironolactone:** Increased risk of severe hyperkalemia.
* **Methotrexate:** Increased risk of bone marrow suppression (additive folate inhibition).
## Monitoring
* **Baseline:** Serum creatinine, electrolytes (especially potassium), and CBC.
* **Ongoing:** Monitor for skin rashes, signs of bone marrow suppression, and renal function. Confirm potassium levels in patients on concomitant RAAS blockers.
## Clinical Pearls
* **Hydration:** Instruct patients to drink plenty of fluids to prevent crystalluria.
* **Sulfonamide Allergy:** May cross-react with other "sulfa" drugs (e.g., sulfonylureas, certain diuretics).
* **Local Resistance:** Resistance patterns for *E. coli* vary significantly; check local antibiograms for empiric UTI therapy.
* **Formulation:** DS tablets (800/160 mg) and SS tablets (400/80 mg). Ensure the prescription clearly specifies the tablet strength.
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**Disclaimer:** This information is for educational purposes only and does not supersede local institutional protocols or official prescribing information. Always verify dosing, contraindications, and drug interactions via a reliable, up-to-date clinical database (e.g., Lexicomp, UpToDate) before prescribing.
