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# Co-Trimoxazole (Septra)
## Overview
Co-trimoxazole is a combination antibiotic consisting of trimethoprim and sulfamethoxazole. It is bactericidal and acts by inhibiting sequential steps in the folic acid synthesis pathway.
## Primary Indications
* Urinary Tract Infections (UTIs)
* *Pneumocystis jirovecii* pneumonia (PCP) prophylaxis and treatment
* Acute otitis media
* Acute exacerbations of chronic bronchitis
* Shigellosis
## Adult Dosing
* **Urinary Tract Infections (Uncomplicated):** 1 double-strength (DS) tablet (160 mg trimethoprim/800 mg sulfamethoxazole) twice daily for 3 days.
* ***Pneumocystis jirovecii* pneumonia (Treatment):** 15-20 mg/kg/day of trimethoprim component divided into 3-4 doses with 75-100 mg/kg/day of sulfamethoxazole component. Administer orally or IV over 60-90 minutes. Duration typically 14-21 days.
* ***Pneumocystis jirovecii* pneumonia (Prophylaxis):** 1 DS tablet once daily, or 1 DS tablet three times weekly. Dosing depends on local protocols.
* **Other Indications:** Dosing varies based on infection and severity. For acute exacerbations of chronic bronchitis, 1 DS tablet twice daily for 14 days is common.
## Pediatric Dosing
* Dosing is calculated based on the trimethoprim component (mg/kg/day) divided into 2-4 doses.
* **Urinary Tract Infections (and other common infections):** 8-10 mg/kg/day of trimethoprim component, divided every 12 hours. Maximum 400 mg trimethoprim component/day.
* ***Pneumocystis jirovecii* pneumonia (Treatment):** 15-20 mg/kg/day of trimethoprim component, divided every 6 hours.
* ***Pneumocystis jirovecii* pneumonia (Prophylaxis):** 5 mg/kg/day of trimethoprim component, divided every 12 hours, given on 3 consecutive days per week. Alternatively, 150 mg/m² of trimethoprim component/day given in 2 divided doses, on 3 consecutive days per week.
## Dose Adjustments
* **Renal Impairment:**
* CrCl > 30 mL/min: No adjustment needed.
* CrCl 15-30 mL/min: Half the usual dose.
* CrCl < 15 mL/min: Avoid use or use with extreme caution, administer only 1/4 to 1/2 the usual dose at 12-hour intervals.
## Contraindications
* Known hypersensitivity to trimethoprim or sulfonamides.
* Documented severe adverse reaction (e.g., Stevens-Johnson syndrome, toxic epidermal necrolysis, aplastic anemia) to sulfonamides or trimethoprim.
* Infants less than 2 months of age (due to risk of kernicterus).
* Megaloblastic anemia due to folate deficiency.
* Severe renal or hepatic insufficiency when pharmacokinetic parameters cannot be monitored.
## Adverse Effects
* **Common:** Rash, nausea, vomiting, diarrhea, anorexia.
* **Serious:** Stevens-Johnson syndrome (SJS), toxic epidermal necrolysis (TEN), severe cutaneous adverse reactions (SCARs), aplastic anemia, agranulocytosis, thrombocytopenia, hyperkalemia, hyponatremia, liver damage, crystalluria (ensure adequate hydration).
## Key Drug Interactions
* **ACE Inhibitors/ARBs/Potassium-Sparing Diuretics:** Increased risk of hyperkalemia.
* **Warfarin:** Increased INR. Monitor INR closely.
* **Methotrexate:** Increased methotrexate levels and toxicity.
* **Digoxin:** Increased digoxin levels.
* **Potassium Supplements:** Increased risk of hyperkalemia.
* **Cyclosporine:** Increased risk of nephrotoxicity.
## Monitoring
* Renal function (creatinine, BUN).
* Electrolytes (especially potassium).
* Complete blood counts (CBC) with differential, particularly during prolonged therapy or in immunocompromised patients.
* Liver function tests.
* Hydration status.
## Clinical Pearls
* Administer with food or milk to minimize gastrointestinal upset.
* Ensure adequate fluid intake to prevent crystalluria.
* Discontinue immediately if rash appears.
* Sulfonamides can displace bilirubin from albumin; caution in newborns and patients with hyperbilirubinemia.
* Use with caution in patients with G6PD deficiency.
* IV formulation should be infused slowly and not mixed with other drugs unless compatibility is established.
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*This information is intended for healthcare professionals and does not substitute for professional medical advice. Always consult the most current prescribing information and clinical guidelines.*
