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# Septran (Co-trimoxazole: Sulfamethoxazole/Trimethoprim)
## Overview
Co-trimoxazole is a fixed-dose combination of a sulfonamide (sulfamethoxazole) and a dihydrofolate reductase inhibitor (trimethoprim) in a 5:1 ratio. It acts synergistically to inhibit sequential steps in bacterial folate synthesis.
## Primary Indications
* Urinary tract infections (UTIs)
* *Pneumocystis jirovecii* pneumonia (PJP) treatment and prophylaxis
* Community-acquired MRSA (skin/soft tissue infections)
* Traveler’s diarrhea
* Stenotrophomonas maltophilia infections
## Adult Dosing
* **UTI (Uncomplicated):** 1 DS tablet (800 mg SMX/160 mg TMP) PO 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:** 1 DS tablet PO daily or 1 SS tablet (400 mg SMX/80 mg TMP) daily.
## Pediatric Dosing
* **General infection:** 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.
* **Note:** Not indicated in infants <2 months of age due to risk of kernicterus.
## Dose Adjustments
* **Renal Impairment (CrCl):**
* 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.
* History of drug-induced immune thrombocytopenia.
* Megaloblastic anemia due to folate deficiency.
* Severe renal or hepatic failure (where monitoring is unavailable).
* Pregnancy at term (risk of kernicterus in the neonate).
## Adverse Effects
* **Common:** Nausea, vomiting, diarrhea, anorexia, skin rash, pruritus.
* **Serious:** Stevens-Johnson syndrome/TEN, severe hyperkalemia, crystalline nephropathy, bone marrow suppression (leukopenia, thrombocytopenia, anemia), and *C. difficile*-associated diarrhea.
## Key Drug Interactions
* **Warfarin:** Significant potentiation of anticoagulant effect (monitor INR).
* **ACE Inhibitors/ARBs/Spironolactone:** Combined risk of severe hyperkalemia.
* **Methotrexate:** Increased risk of bone marrow suppression/pancytopenia.
* **Phenytoin:** Increased serum concentrations of phenytoin.
* **Dofetilide:** Contraindicated; risk of QT prolongation and life-threatening arrhythmias.
## Monitoring
* **Renal/Electrolytes:** Baseline and periodic serum creatinine and potassium (especially in elderly or patients on ACEIs/ARBs).
* **Hematology:** CBC (especially during prolonged therapy >14 days) to monitor for blood dyscrasias.
* **Clinical:** Watch for signs of severe cutaneous adverse reactions (rash, blistering).
## Clinical Pearls
* **Hydration:** Maintain adequate fluid intake to prevent crystalluria.
* **Bioavailability:** PO formulation is near 100% bioavailable; IV-to-PO conversion is typically 1:1.
* **Sulfonamide Allergy:** Cross-reactivity with non-sulfonamide antibiotics is considered rare, but exercise caution in patients with history of severe hypersensitivity reactions (e.g., SJS/TEN).
* **Protocol Variance:** Always verify dosing against local institutional antibiograms, as resistance patterns (specifically for *E. coli*) vary widely by geography.
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**Educational Disclaimer:** This information is for educational purposes only. Drug dosing, local resistance patterns, and guidelines change frequently. Always consult your current hospital formulary, clinical pharmacists, and official prescribing information (package insert) before administering medications.
