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# Septran (Co-trimoxazole: Sulfamethoxazole/Trimethoprim)
## Overview
A fixed-dose combination antibiotic consisting 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 and soft tissue infections
* Acute otitis media (pediatric)
* Traveler’s diarrhea / Shigellosis
## Adult Dosing
* **Mild/Moderate Infection:** 160/800 mg (double strength) every 12 hours.
* **PJP Treatment:** 15–20 mg/kg/day (based on TMP component) divided every 6–8 hours.
* **PJP Prophylaxis:** 160/800 mg once daily or thrice weekly.
## Pediatric Dosing (>2 months)
* **Mild/Moderate 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.
* **PJP Prophylaxis:** 150 mg/m²/day (based on TMP) divided every 12 hours (max 320 mg TMP/day) administered 3 consecutive days/week.
## Dose Adjustments
* **Renal Impairment:** If CrCl 15–30 mL/min, reduce dose by 50%. Co-trimoxazole is not recommended if CrCl <15 mL/min.
* **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.
* Pregnancy at term (risk of kernicterus).
* Severe renal or hepatic impairment (where monitoring is impossible).
## Adverse Effects
* **Common:** Nausea, vomiting, rash, pruritus.
* **Serious:** Stevens-Johnson syndrome/TEN, bone marrow suppression (leukopenia, thrombocytopenia), hyperkalemia, crystalluria (maintain hydration), and *Clostridioides difficile*-associated diarrhea.
## Key Drug Interactions
* **Warfarin:** Significant increase in INR; monitor closely.
* **ACE Inhibitors/ARBs/Potassium-sparing diuretics:** Increased risk of hyperkalemia.
* **Methotrexate:** Increased risk of myelosuppression and nephrotoxicity.
* **Cyclosporine/Tacrolimus:** Increased risk of nephrotoxicity.
## Monitoring
* Renal function (BUN/SCr) and electrolytes (especially potassium).
* Complete blood count (CBC) during prolonged therapy.
* Signs of hypersensitivity or skin reactions.
* INR if on warfarin.
## Clinical Pearls
* **Hydration:** Instruct patients to drink plenty of water to prevent crystalluria.
* **Photosensitivity:** Advise patients to use sun protection.
* **Dosing note:** Doses are frequently referred to by the trimethoprim (TMP) component. Always verify if the order refers to total dose or TMP component.
* **Local resistance:** High rates of *E. coli* resistance have emerged in many regions; consult local antibiograms for empiric UTI selection.
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*Disclaimer: This information is for educational purposes only. Clinical practice guidelines and local resistance patterns vary. Always verify specific dosing, safety, and compatibility information through current institutional protocols, the package insert, or professional clinical databases (e.g., Lexicomp, Micromedex) before prescribing or administering medication.*
