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# Co-trimoxazole (Septra)
## Overview
Co-trimoxazole is a combination antibiotic containing sulfamethoxazole and trimethoprim.
## Primary Indications
* Treatment of urinary tract infections (UTIs)
* Treatment of *Pneumocystis jirovecii* pneumonia (PJP)
* Treatment of shigellosis
* Prophylaxis against PJP in immunocompromised patients
## Adult Dosing
* **Urinary Tract Infections:** 1 double-strength (DS) tablet (800 mg sulfamethoxazole/160 mg trimethoprim) twice daily for 3-14 days.
* ***Pneumocystis jirovecii* Pneumonia (Treatment):** 15 mg/kg/day of trimethoprim component, divided into 3-4 doses, for 21 days. This typically translates to 2 DS tablets every 8 hours.
* ***Pneumocystis jirovecii* Pneumonia (Prophylaxis):** 1 DS tablet once daily, or 1 DS tablet three times weekly.
* **Shigellosis:** 1 DS tablet twice daily for 5-7 days.
## Pediatric Dosing
Dosing is based on the trimethoprim component.
* **Urinary Tract Infections:** 8 mg/kg/day of trimethoprim, divided into two doses, given every 12 hours. This is typically 1 DS tablet twice daily for children over 12 years, and adjusted for younger children based on weight.
* ***Pneumocystis jirovecii* Pneumonia (Treatment):** 15 mg/kg/day of trimethoprim, divided into 3-4 doses, for 21 days.
* ***Pneumocystis jirovecii* Pneumonia (Prophylaxis):** 150 mg/m²/day of trimethoprim, divided into two doses, for 3 consecutive days per week.
## Dose Adjustments
* **Renal Impairment (Adults):**
* CrCl > 30 mL/min: No adjustment needed.
* CrCl 15-30 mL/min: Give usual dose every 12 hours.
* CrCl < 15 mL/min: Avoid use. If essential, give 1/2 to 1/4 of the usual dose every 12 hours.
## Contraindications
* Hypersensitivity to trimethoprim or sulfonamides.
* Documented megaloblastic anemia due to folate deficiency.
* Infants less than 2 months of age (risk of kernicterus).
* Severe renal or hepatic impairment.
## Adverse Effects
Common: Rash, pruritus, nausea, vomiting, diarrhea.
Serious: Stevens-Johnson syndrome, toxic epidermal necrolysis, hyperkalemia, bone marrow suppression (anemia, leukopenia, thrombocytopenia), crystalluria, photosensitivity, hepatic dysfunction.
## Key Drug Interactions
* **Warfarin:** Increased INR. Monitor INR closely.
* **ACE inhibitors, ARBs, potassium-sparing diuretics, spironolactone:** Increased risk of hyperkalemia.
* **Methotrexate:** Increased risk of methotrexate toxicity (bone marrow suppression, renal toxicity).
* **Phenytoin:** May increase phenytoin levels.
* **Potassium supplements:** Increased risk of hyperkalemia.
## Monitoring
* Renal function (SCr, CrCl).
* Electrolytes (especially potassium).
* Complete blood count (CBC) with differential, particularly with prolonged therapy or in immunocompromised patients.
* Signs of hypersensitivity reactions and skin reactions.
## Clinical Pearls
* Co-trimoxazole can crystallize in the urine; ensure adequate fluid intake, especially during treatment for UTIs.
* Photosensitivity reactions can occur; advise patients to use sun protection.
* Sulfonamides can displace bilirubin from albumin, posing a risk of kernicterus in neonates.
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*Disclaimer: This information is intended for clinical decision support and does not replace professional judgment or current prescribing information. Always verify with the most up-to-date drug product labeling and institutional protocols.*
