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# Septran (Co-Trimoxazole / Trimethoprim-Sulfamethoxazole)
## Overview
Co-trimoxazole is a synergistic combination of trimethoprim (a dihydrofolate reductase inhibitor) and sulfamethoxazole (a sulfonamide) in a fixed 1:5 ratio. It is bactericidal against a broad spectrum of Gram-positive and Gram-negative organisms.
## Primary Indications
* Urinary tract infections (UTI)
* *Pneumocystis jirovecii* pneumonia (PJP/PCP) treatment and prophylaxis
* Stenotrophomonas maltophilia infections
* MRSA skin and soft tissue infections
* Traveler's diarrhea
## Adult Dosing
* **UTI (Uncomplicated):** 160 mg TMP/800 mg SMX (DS 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 mg TMP/800 mg SMX once daily or three times weekly.
## Pediatric Dosing (Age >2 months)
* **General Infections:** 8–10 mg/kg/day of TMP component divided every 12 hours.
* **PJP Treatment:** 15–20 mg/kg/day of TMP component divided every 6 hours.
* **PJP Prophylaxis:** 150 mg/m²/day of TMP component divided twice daily, 3 days a week.
## Dose Adjustments
* **Renal Impairment (CrCl):**
* 30–50 mL/min: Reduce dose by 25–50%.
* 15–30 mL/min: Reduce dose by 50%.
* <15 mL/min: Not recommended (unless hemodialysis).
* **Hepatic Impairment:** Use with caution; monitor for hepatotoxicity.
## Contraindications
* Hypersensitivity to sulfonamides or trimethoprim.
* Documented megaloblastic anemia due to folate deficiency.
* Marked hepatic or severe renal impairment.
* Pregnancy at term (risk of kernicterus in neonates).
* Infants <2 months old (risk of kernicterus).
## Adverse Effects
* **Common:** Nausea, vomiting, skin rash, pruritus.
* **Serious:** Stevens-Johnson syndrome (SJS), Toxic Epidermal Necrolysis (TEN), agranulocytosis, hyperkalemia (dose-dependent), acute kidney injury (interstitial nephritis), and *Clostridioides difficile* infection.
## Key Drug Interactions
* **Warfarin:** Significant increase in INR; monitor closely.
* **ACE Inhibitors/ARBs/Spironolactone:** Increased risk of severe hyperkalemia.
* **Methotrexate:** Increased risk of myelosuppression.
* **CYP2C9 Inhibitors/Substrates:** May increase plasma concentrations of phenytoin.
## Monitoring
* **Renal Function:** Serum creatinine and electrolytes (specifically potassium).
* **Hematology:** CBC with differential (monitor for myelosuppression during long-term therapy).
* **Clinical:** Watch for signs of severe cutaneous adverse reactions (rash, blistering, mucosal involvement).
## Clinical Pearls
* **Hydration:** Maintain adequate fluid intake to prevent crystalluria.
* **Solubility:** Sulfamethoxazole can precipitate in the urine; ensure the patient is well-hydrated.
* **Hyperkalemia:** Particularly relevant in elderly patients or those on concurrent RAAS inhibitors.
* **Dosing Weight:** In PJP treatment, dosing is almost exclusively based on the TMP component.
* **Local Protocols:** Significant resistance patterns (e.g., *E. coli*) vary geographically. Always consult local antibiograms.
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**Educational Disclaimer:** This information is for educational purposes only. Drug dosing, contraindications, and interaction profiles may change. Always verify current prescribing information using official drug monographs or institutional clinical protocols before prescribing or administering medication.
