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## Septran (Co-trimoxazole)
### Overview
Co-trimoxazole is a combination antibiotic of trimethoprim and sulfamethoxazole. It works by inhibiting sequential steps in the bacterial folic acid synthesis pathway.
### Primary Indications
* Urinary tract infections (UTIs)
* Pneumocystis jirovecii pneumonia (PJP) prophylaxis and treatment
* *Shigella* enteritis
* *Haemophilus influenzae* otitis media
* Certain respiratory tract infections
### Adult Dosing
* **General UTIs:** 1 double-strength (DS) tablet (160 mg trimethoprim/800 mg sulfamethoxazole) orally every 12 hours for 3-7 days.
* **PJP Prophylaxis:** 1 DS tablet orally once daily.
* **PJP Treatment:** 15-20 mg/kg/day (trimethoprim component) divided into 3-4 doses (e.g., 2 DS tablets every 6-8 hours) IV or orally for 14-21 days.
* **Shigellosis:** 1 DS tablet orally every 12 hours for 5 days.
* Maximum daily dose typically 4 DS tablets.
### Pediatric Dosing
Dosing is based on the trimethoprim component, usually 5-10 mg/kg/day (mild-to-moderate infections) or 15-20 mg/kg/day (severe infections) divided into 2-4 doses. Sulfamethoxazole dose is 5 times the trimethoprim dose.
* **General Infections:** 5-10 mg/kg/day of trimethoprim divided every 12 hours.
* **PJP Prophylaxis:** 5 mg/kg/day of trimethoprim divided every 12 hours, 3 times a week.
* **PJP Treatment:** 15-20 mg/kg/day of trimethoprim divided every 6-8 hours.
*Exact pediatric dosing should follow local protocols or specific guidelines due to the varied indications and severity.*
### Dose Adjustments
* **Renal Impairment:**
* CrCl > 30 mL/min: No adjustment needed.
* CrCl 15-30 mL/min: Give usual dose every 12-18 hours.
* CrCl < 15 mL/min: Avoid use or give usual dose every 24 hours.
* **Hepatic Impairment:** Use with caution; no specific dosing adjustment.
### Contraindications
* Known hypersensitivity to trimethoprim or sulfonamides.
* Documented megaloblastic anemia due to folate deficiency.
* Infants < 2 months of age (risk of kernicterus).
* Severe renal or hepatic insufficiency where drug levels cannot be monitored.
* History of drug-induced immune thrombocytopenia with previous use of trimethoprim or sulfonamides.
### Adverse Effects
Common: Nausea, vomiting, rash (including Stevens-Johnson syndrome, toxic epidermal necrolysis).
Less common: Diarrhea, pruritus, urticaria, hyperkalemia, leukopenia, neutropenia, thrombocytopenia, eosinophilia, crystalluria.
Rare: Hepatitis, interstitial nephritis, photosensitivity, hallucinations, depression.
### Key Drug Interactions
* **Warfarin:** Increased INR; monitor INR closely and adjust warfarin dose.
* **ACE Inhibitors/ARBs/Potassium-Sparing Diuretics:** Increased risk of hyperkalemia.
* **Methotrexate:** Increased risk of bone marrow suppression.
* **CYP2C9 Substrates (e.g., phenytoin, tolbutamide):** Increased plasma concentrations.
* **Dapsone:** Increased dapsone levels; monitor for methemoglobinemia.
* **Potassium Supplements/Potassium-Containing Salt Substitutes:** Increased risk of hyperkalemia.
* **Rifampin:** May decrease co-trimoxazole levels.
### Monitoring
* Complete blood counts (CBC) with differential and platelet count at baseline and periodically during treatment, especially in prolonged therapy or immunocompromised patients.
* Renal and hepatic function tests.
* Electrolytes, especially potassium, particularly in patients with renal impairment or on ACE inhibitors/ARBs.
* For prolonged use, monitor folate levels.
* Signs of hypersensitivity reactions.
### Clinical Pearls
* Encourage increased fluid intake to prevent crystalluria, especially during IV administration.
* Monitor for signs of hematologic toxicity and rash closely. Discontinue immediately if rash appears.
* Co-trimoxazole can interfere with certain urine glucose tests.
* For PJP prophylaxis in HIV-infected patients, preferred regimens are 1 DS tablet daily or 1 SS tablet twice daily.
***
*Disclaimer: This information is intended for healthcare professionals. Always consult the most current prescribing information and relevant guidelines for complete details before making any clinical decisions.*
