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# Co-trimoxazole (Septran)
## Overview
Co-trimoxazole is a fixed-dose combination of sulfamethoxazole (a sulfonamide antibiotic) and trimethoprim (a dihydrofolate reductase inhibitor). It exhibits synergistic activity by inhibiting sequential steps in the folic acid synthesis pathway, which is essential for bacterial growth.
## Primary Indications
* Urinary tract infections (UTIs) due to susceptible organisms.
* *Pneumocystis jirovecii* pneumonia (PJP) prophylaxis and treatment.
* Infections due to *Staphylococcus aureus* (including methicillin-resistant *Staphylococcus aureus*, MRSA) in sites where it is susceptible.
* Traveler's diarrhea.
* Other susceptible bacterial infections.
## Adult Dosing
* **UTIs (uncomplicated):** 1 DS (double strength) tablet (160 mg trimethoprim/800 mg sulfamethoxazole) every 12 hours for 3 days.
* **UTIs (complicated):** 1 DS tablet every 12 hours for 7-14 days.
* **PJP Treatment:** 15 mg/kg/day (based on trimethoprim component) divided into 3 or 4 doses IV or PO for 14-21 days.
* **PJP Prophylaxis:** 1 DS tablet once daily, or 1 DS tablet 3 times weekly.
* **Traveler's Diarrhea:** 1 DS tablet every 12 hours for 5 days.
* **MRSA (skin and soft tissue infections):** Dosing varies; consult local protocols or infectious disease specialists. Often 1-2 DS tablets every 12 hours.
* **Maximum dose:** Generally, no more than 8 DS tablets (640 mg trimethoprim/3200 mg sulfamethoxazole) per day.
## Pediatric Dosing
Dosing is typically calculated based on the trimethoprim component (5-20 mg/kg/day) divided into 2-4 doses, depending on the indication. Sulfamethoxazole dose is adjusted proportionally (35-100 mg/kg/day).
* **UTIs (children > 2 months):** 8 mg/kg/day trimethoprim and 40 mg/kg/day sulfamethoxazole, divided into two doses every 12 hours.
* **PJP Treatment (children > 2 months):** 20 mg/kg/day trimethoprim and 100 mg/kg/day sulfamethoxazole, divided into 4 doses every 6 hours for 14-21 days.
* **PJP Prophylaxis (children > 2 months):** 5 mg/kg/day trimethoprim and 25 mg/kg/day sulfamethoxazole, divided into two doses daily, or 10 mg/kg/day trimethoprim and 50 mg/kg/day sulfamethoxazole, divided into two doses every 12 hours, 3 times a week.
**Note:** Specific pediatric dosing protocols may vary; consult local guidelines.
## Dose Adjustments
* **Renal Impairment (CrCl < 15 mL/min):** Avoid use.
* **Renal Impairment (CrCl 15-30 mL/min):** Reduce dose by 50%.
* **Renal Impairment (CrCl 30-45 mL/min):** Reduce dose by 25%.
* **Hepatic Impairment:** Use with caution; dose adjustments may be needed.
## Contraindications
* Known hypersensitivity to trimethoprim, sulfamethoxazole, or sulfonamides.
* Infants < 2 months of age (risk of kernicterus).
* Marked liver parenchymal damage.
* Anuria or oliguria.
* Megaloblastic anemia due to folate deficiency.
* Pregnancy at term and breastfeeding mothers (risk of kernicterus).
## Adverse Effects
* **Common:** Nausea, vomiting, rash (including Stevens-Johnson syndrome and toxic epidermal necrolysis, though rare), hyperkalemia, increased serum creatinine.
* **Serious:** Hematologic abnormalities (anemia, leukopenia, thrombocytopenia, agranulocytosis), hypersensitivity reactions, crystalluria, photosensitivity, liver injury, C. difficile-associated diarrhea.
## Key Drug Interactions
* **ACE Inhibitors and ARBs:** Increased risk of hyperkalemia.
* **Diuretics (especially thiazides):** Increased risk of hyperkalemia and thrombocytopenia, particularly in elderly patients.
* **Warfarin:** Increased INR. Monitor INR closely and adjust warfarin dose as needed.
* **Methotrexate:** Increased risk of methotrexate toxicity (myelosuppression, megaloblastic anemia).
* **Potassium-sparing diuretics:** Increased risk of hyperkalemia.
* **Cyclosporine:** Increased cyclosporine levels, potentially leading to nephrotoxicity.
* **Digoxin:** Increased digoxin levels.
* **Oral hypoglycemics:** Potentiated hypoglycemic effect.
## Monitoring
* **Renal function:** Monitor serum creatinine and BUN, especially in patients with pre-existing renal disease or those receiving concurrent nephrotoxic agents.
* **Hematologic parameters:** Complete blood counts (CBC) with differential should be monitored, especially during prolonged therapy or in immunocompromised patients.
* **Electrolytes:** Monitor potassium levels, especially in patients with renal impairment or those taking other medications that affect potassium.
* **Liver function tests:** Monitor if liver dysfunction is suspected.
* **Hydration:** Ensure adequate fluid intake to prevent crystalluria.
## Clinical Pearls
* Co-trimoxazole should be taken with a full glass of water and patients should be encouraged to maintain adequate fluid intake to prevent crystalluria.
* Photosensitivity reactions can occur; advise patients to use sun protection.
* The risk of hypersensitivity reactions is higher in patients with a history of sulfonamide allergy.
* For IV administration, dilute the solution and administer slowly to avoid local irritation and phlebitis.
* Always confirm susceptibility testing for indications where resistance is common (e.g., UTIs, *Staphylococcus aureus*).
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*This information is intended for clinical professionals and is not a substitute for detailed product labeling or local institutional protocols. Always consult the most current prescribing information and consider individual patient factors.*
