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## Septra (Co-trimoxazole)
### Overview
Septra is a combination antibiotic consisting of trimethoprim and sulfamethoxazole. It is a bacteriostatic agent that inhibits folic acid synthesis.
### Primary Indications
* Urinary tract infections (UTIs)
* Pneumocystis jirovecii pneumonia (PJP) prophylaxis and treatment
* Acute exacerbations of chronic bronchitis
* Shigellosis
* Traveler's diarrhea (ETEC)
### Adult Dosing
* **UTIs, Acute exacerbations of chronic bronchitis, Shigellosis, Traveler's diarrhea:** 1 double-strength (DS) tablet (160 mg trimethoprim/800 mg sulfamethoxazole) every 12 hours. Duration varies by indication.
* **PJP Prophylaxis (in immunocompromised patients):** 1 DS tablet once daily.
* **PJP Treatment:** 15 mg/kg/day of trimethoprim component, given in 3 or 4 divided doses, typically for 14-21 days. This is equivalent to approximately 3 DS tablets every 8 hours.
### Pediatric Dosing
Dosing is based on the trimethoprim component.
* **UTIs, Shigellosis, Traveler's diarrhea:** 8 mg/kg/day of trimethoprim component, divided into two doses, given every 12 hours.
* This corresponds to approximately 20 mg/kg/day of co-trimoxazole.
* Example: For a 20 kg child, the dose would be 160 mg trimethoprim/800 mg sulfamethoxazole (1 DS tablet) every 12 hours.
* **PJP Prophylaxis:** 5 mg/kg/day of trimethoprim component, divided into two doses, given every 12 hours.
* This corresponds to approximately 10 mg/kg/day of co-trimoxazole.
* Administer at 1 DS tablet daily or 2 DS tablets three times a week, depending on protocol.
* **PJP Treatment:** 15 mg/kg/day of trimethoprim component, divided into 3 or 4 doses, every 6 or 8 hours.
*Note: Pediatric dosing for children under 2 months is generally not recommended due to the risk of kernicterus.*
### Dose Adjustments
* **Renal Impairment (Adults):**
* CrCl > 30 mL/min: No adjustment needed.
* CrCl 15-30 mL/min: Give usual dose every 18 hours.
* CrCl < 15 mL/min: Use with caution, or avoid. If used, give usual dose every 24 hours or reduce dose by 50-75%.
* **Hepatic Impairment:** Use with caution.
### Contraindications
* Known hypersensitivity to trimethoprim, sulfamethoxazole, or sulfonamides.
* Documented megaloblastic anemia due to folate deficiency.
* Infants less than 2 months of age.
* Severe renal insufficiency when repeated laboratory tests cannot be performed.
* Severe hepatic insufficiency.
### Adverse Effects
Common: Rash, nausea, vomiting, diarrhea, hyperkalemia.
Serious: Severe cutaneous reactions (Stevens-Johnson syndrome, toxic epidermal necrolysis), hematologic toxicity (anemia, leukopenia, thrombocytopenia), hypersensitivity reactions, hyponatremia, hyperkalemia, renal impairment, hepatic injury, photosensitivity.
### Key Drug Interactions
* **ACE Inhibitors/ARBs/Potassium-Sparing Diuretics:** Increased risk of hyperkalemia.
* **Warfarin:** Increased anticoagulant effect. Monitor INR closely.
* **Methotrexate:** Increased methotrexate levels and toxicity.
* **Digoxin:** Increased digoxin levels.
* **Thiazide Diuretics:** Increased risk of thrombocytopenia in elderly patients.
* **Potassium Supplements:** Increased risk of hyperkalemia.
* **CYP2C9 Substrates:** May increase levels of drugs metabolized by CYP2C9.
### Monitoring
* Complete blood counts (CBC) with differential, especially with prolonged therapy or in elderly patients.
* Renal function (BUN, creatinine).
* Liver function tests.
* Electrolytes (especially potassium).
* Skin for rash.
* INR if patient is on warfarin.
### Clinical Pearls
* Advise patients to drink plenty of fluids to prevent crystalluria.
* Rash can occur at any time during treatment; discontinue if severe.
* Sulfonamides can cause a "red-man" like reaction if given intravenously too rapidly (though less common with oral formulations).
* Consider folate supplementation for patients on long-term therapy, especially those with risk factors for deficiency.
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*This information is intended for healthcare professionals and is not a substitute for professional medical advice. Always consult the most current prescribing information and clinical guidelines for complete details.*
