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# Septran (Co-trimoxazole)
## Overview
Co-trimoxazole is a combination antibiotic of trimethoprim and sulfamethoxazole, typically in an 8:1 ratio. It inhibits sequential steps in bacterial folic acid synthesis.
## Primary Indications
* Urinary tract infections (UTIs)
* *Pneumocystis jirovecii* pneumonia (PCP) prophylaxis and treatment
* *Shigellosis*
* *Stenotrophomonas maltophilia* infections
* Other susceptible bacterial infections
## Adult Dosing
* **Uncomplicated UTI:** 1 DS (double strength) tablet (160 mg trimethoprim/800 mg sulfamethoxazole) every 12 hours for 3-5 days.
* **Complicated UTI/Shigellosis:** 1 DS tablet every 12 hours for 7-14 days.
* **PCP Treatment:** 15 mg/kg/day trimethoprim component divided every 6 hours, with a maximum of 960 mg trimethoprim (2 DS tablets) every 6 hours. Typically given with a corticosteroid in moderate to severe cases. Duration is 14-21 days.
* **PCP Prophylaxis:** 1 DS tablet daily, or 1 DS tablet three times weekly.
* **Stenotrophomonas maltophilia:** Dosing varies; typically 15-20 mg/kg/day trimethoprim component divided every 12 hours.
## Pediatric Dosing
Dosing is based on the trimethoprim component.
* **UTI/Shigellosis:** 8 mg/kg/day trimethoprim component divided every 12 hours.
* **PCP Prophylaxis:** 5 mg/kg/day trimethoprim component divided every 12 hours, or 150 mg/m²/day divided every 12 hours.
* **PCP Treatment:** 15 mg/kg/day trimethoprim component divided every 6 hours.
Dosing depends on age and weight, and may require specific weight-based calculations.
## Dose Adjustments
* **Renal Impairment (CrCl < 15 mL/min):** Avoid use.
* **Renal Impairment (CrCl 15-30 mL/min):** Use half the usual dose.
* **Renal Impairment (CrCl 30-45 mL/min):** Use 3/4 the usual dose.
## Contraindications
* Known hypersensitivity to trimethoprim or sulfonamides
* History of drug-induced thrombocytopenia with trimethoprim or sulfonamides
* Marked vitamin B12 deficiency
* Megaloblastic anemia due to folate deficiency
* Infants < 2 months of age (risk of kernicterus)
* Severe renal or hepatic insufficiency when plasma concentrations cannot be monitored
## Adverse Effects
* **Common:** Rash, nausea, vomiting, diarrhea, anorexia.
* **Serious:** Stevens-Johnson syndrome, toxic epidermal necrolysis, severe cutaneous reactions, hyperkalemia, hyponatremia, bone marrow suppression (anemia, leukopenia, thrombocytopenia), hepatic dysfunction, photosensitivity, crystalluria.
## Key Drug Interactions
* **ACE inhibitors/ARBs/Potassium-sparing diuretics:** Increased risk of hyperkalemia.
* **Warfarin:** Increased INR; monitor INR closely.
* **Methotrexate:** Increased methotrexate toxicity; may increase risk of bone marrow suppression.
* **Cyclosporine:** Increased risk of nephrotoxicity.
* **Sulfonylureas:** Increased risk of hypoglycemia.
* **Potassium supplements:** Increased risk of hyperkalemia.
* **Diuretics (thiazide, loop):** Increased risk of thrombocytopenia in elderly patients.
## Monitoring
* **Renal function:** Crucial, especially in elderly or those with pre-existing renal disease.
* **Electrolytes:** Particularly potassium.
* **Complete blood counts (CBCs):** With differential, especially with prolonged therapy or in immunocompromised patients.
* **Hepatic function tests.**
* **Signs of hypersensitivity reactions:** Skin rashes, fever.
## Clinical Pearls
* Administer with food or milk to minimize GI upset.
* Ensure adequate fluid intake to prevent crystalluria.
* Avoid in G6PD deficiency if possible due to risk of hemolysis.
* Higher doses and prolonged therapy increase the risk of adverse effects, particularly hematologic and dermatologic reactions.
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*Disclaimer: This information is intended for healthcare professionals and is not a substitute for professional medical advice. Always consult the most current prescribing information and relevant guidelines before making clinical decisions.*
