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## Septra (Trimethoprim/Sulfamethoxazole)
### Overview
Septra is a combination antibiotic containing trimethoprim and sulfamethoxazole in a 1:5 ratio. 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 enteritis
### Adult Dosing
* **Uncomplicated UTIs:** 1 double-strength (DS) tablet (160 mg trimethoprim/800 mg sulfamethoxazole) every 12 hours for 3 days.
* **PJP Treatment:** 15-20 mg/kg/day of trimethoprim component, divided into 3-4 doses, administered orally or IV, for 14-21 days.
* **PJP Prophylaxis:** 1 DS tablet daily.
* **Other Indications:** Dosing varies based on the specific infection. A common regimen for other indications is 1-2 DS tablets every 12 hours.
### Pediatric Dosing
* Dosing is based on the trimethoprim component: 8-10 mg/kg/day for UTIs and PJP treatment, divided into two doses.
* For PJP prophylaxis, 5 mg/kg/day of trimethoprim component, divided into two doses.
* For infants under 2 months, trimethoprim/sulfamethoxazole is generally **contraindicated** due to the risk of kernicterus.
### Dose Adjustments
* **Renal Impairment:**
* CrCl > 30 mL/min: No adjustment necessary.
* CrCl 15-30 mL/min: Administer 75% of the standard dose.
* CrCl < 15 mL/min: Avoid use or administer 50% of the standard dose every 12-36 hours, with close monitoring.
### Contraindications
* Hypersensitivity to trimethoprim or sulfonamides
* Megaloblastic anemia due to folate deficiency
* Infants younger than 2 months
* Severe renal or hepatic insufficiency where urine or blood levels cannot be monitored
### Adverse Effects
* Gastrointestinal: Nausea, vomiting, diarrhea
* Dermatologic: Rash, Stevens-Johnson syndrome, toxic epidermal necrolysis
* Hematologic: Agranulocytosis, aplastic anemia, thrombocytopenia, leukopenia (especially with prolonged use)
* Renal: Crystalluria (ensure adequate hydration)
* Hepatic: Elevated liver enzymes, hepatitis
### Key Drug Interactions
* **Warfarin:** Increased INR. Monitor INR closely and adjust warfarin dose as needed.
* **ACE inhibitors/ARBs/Potassium-sparing diuretics:** Increased risk of hyperkalemia.
* **Methotrexate:** Increased methotrexate levels and toxicity.
* **Cyclosporine:** Increased cyclosporine levels.
* **Potassium:** Increased risk of hyperkalemia, especially with other potassium-increasing agents.
* **Digoxin:** Increased digoxin levels.
### Monitoring
* Renal function (BUN, creatinine)
* Complete blood counts (CBC) with differential, especially with prolonged therapy or in immunocompromised patients.
* Liver function tests
* Electrolytes (potassium)
* Signs of hypersensitivity reactions
### Clinical Pearls
* Ensure adequate fluid intake to prevent crystalluria.
* Administer with food to minimize gastrointestinal upset.
* Use with caution in patients with G6PD deficiency, as sulfonamides can cause hemolytic anemia.
* Be aware of cross-reactivity with other sulfonamide-containing medications.
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**Disclaimer:** This information is intended for clinical professionals. Always consult the most current prescribing information and local guidelines for definitive dosing and management.
