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# Co-Trimoxazole (Septra, Bactrim)
## Overview
Co-trimoxazole is a combination antibiotic of trimethoprim and sulfamethoxazole.
## Primary Indications
* Urinary tract infections (UTIs)
* Pneumocystis jirovecii pneumonia (PCP) prophylaxis and treatment
* *Shigella* enteritis
* Acute otitis media
* Traveler's diarrhea
* Certain respiratory tract infections
## Adult Dosing
Dosing varies significantly by indication. A common regimen for uncomplicated UTIs is 1 double-strength (DS) tablet (800 mg sulfamethoxazole/160 mg trimethoprim) every 12 hours for 3 days. For PCP treatment, doses are higher (e.g., 15 mg/kg/day trimethoprim component, divided every 6 hours). Refer to specific guidelines for definitive dosing.
## Pediatric Dosing
Dosing is based on the trimethoprim component, typically 5-10 mg/kg/day divided every 12 hours for mild-to-moderate infections, and up to 20 mg/kg/day divided every 6 hours for severe infections or PCP treatment. For PCP prophylaxis, 5 mg/kg/day divided every 12 hours. Consult specific guidelines.
## Dose Adjustments
* **Renal Impairment:** Adjust dose based on creatinine clearance (CrCl). If CrCl > 30 mL/min, use usual dose. If CrCl 15-30 mL/min, use 1/2 usual dose. If CrCl < 15 mL/min, avoid use or use 1/4 usual dose.
## Contraindications
* Hypersensitivity to trimethoprim or sulfonamides
* Documented severe adverse reaction to sulfonamides (e.g., Stevens-Johnson syndrome)
* Infants less than 2 months old (risk of kernicterus)
* Marked liver impairment
* Severe renal insufficiency when plasma concentrations cannot be monitored
## Adverse Effects
Common: Nausea, vomiting, rash, hyperkalemia.
Serious: Stevens-Johnson syndrome, toxic epidermal necrolysis, aplastic anemia, agranulocytosis, severe rash (e.g., DRESS), photosensitivity, hyponatremia, hyperkalemia, renal damage (crystalluria), hepatic dysfunction.
## Key Drug Interactions
* **Warfarin:** Increased INR. Monitor INR closely.
* **ACE inhibitors/ARBs/Potassium-sparing diuretics:** Increased risk of hyperkalemia.
* **Methotrexate:** Increased methotrexate levels and toxicity.
* **Sulfonylureas:** Increased hypoglycemic effect.
* **Thiazide diuretics:** Increased risk of hyperkalemia, particularly in elderly patients.
* **Potassium supplements:** Increased risk of hyperkalemia.
## Monitoring
* Renal function (BUN, creatinine)
* Electrolytes (especially potassium)
* Complete blood counts (CBC) with differential, particularly with prolonged therapy or in immunocompromised patients.
* Liver function tests
* Signs of hypersensitivity reactions (rash, fever)
## Clinical Pearls
* Ensure adequate hydration to prevent crystalluria.
* Advise patients to report any rash immediately.
* Use with caution in patients with G6PD deficiency.
* Not recommended for routine use in pregnancy, especially in the third trimester, due to potential risk to the fetus.
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*Disclaimer: This information is for educational purposes only and does not substitute for professional medical advice. Always consult with a qualified healthcare provider and verify current prescribing information before making any treatment decisions.*
