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# Co-trimoxazole (Sulfamethoxazole/Trimethoprim)
## Overview
Co-trimoxazole is a combination antibiotic consisting of sulfamethoxazole and trimethoprim in a 5:1 ratio. It exhibits a broad spectrum of activity against many gram-positive and gram-negative bacteria.
## Primary Indications
* **Urinary Tract Infections (UTIs):** Acute uncomplicated UTIs, prophylaxis and treatment of recurrent UTIs.
* **Pneumocystis jirovecii Pneumonia (PJP):** Prophylaxis and treatment.
* **Bacterial Infections:** Including otitis media, acute exacerbations of chronic bronchitis, and Shigella gastroenteritis.
## Adult Dosing
* **Urinary Tract Infections (Acute Uncomplicated):** 1 double-strength (DS) tablet (160 mg trimethoprim/800 mg sulfamethoxazole) every 12 hours for 3 days.
* **Urinary Tract Infections (Complicated or Prophylaxis):** 1 DS tablet every 12 hours. Duration varies by indication.
* **Pneumocystis jirovecii Pneumonia (Treatment):** 15-20 mg/kg/day of trimethoprim component divided into 3-4 doses every 12-24 hours for 14-21 days.
* **Pneumocystis jirovecii Pneumonia (Prophylaxis):** 1 DS tablet once daily, or 1 DS tablet three times weekly.
* **Other Bacterial Infections:** Dosing varies based on severity and site of infection. A common regimen is 1-2 DS tablets every 12 hours. Maximum daily dose of trimethoprim should generally not exceed 20 mg/kg.
## Pediatric Dosing
* **Urinary Tract Infections:** 8 mg/kg/day of trimethoprim component and 40 mg/kg/day of sulfamethoxazole component, given in two equally divided doses every 12 hours. For children over 12 years, adult dosing can be used.
* **Pneumocystis jirovecii Pneumonia (Treatment):** 15-20 mg/kg/day of trimethoprim component and 75-100 mg/kg/day of sulfamethoxazole component, given in three or four equally divided doses every 6-8 hours for 14-21 days.
* **Pneumocystis jirovecii Pneumonia (Prophylaxis):** 5 mg/kg/day of trimethoprim component and 25 mg/kg/day of sulfamethoxazole component, given once daily.
## Dose Adjustments
* **Renal Impairment:** Adjustments are necessary based on creatinine clearance (CrCl).
* CrCl > 30 mL/min: No adjustment needed.
* CrCl 15-30 mL/min: Half the usual dose.
* CrCl < 15 mL/min: Avoid use.
* **Hepatic Impairment:** Use with caution.
## Contraindications
* Known hypersensitivity to trimethoprim, sulfonamides, or any component of the formulation.
* Documented megaloblastic anemia due to folate deficiency.
* Infants less than 2 months of age (due to risk of kernicterus).
* Severe renal insufficiency in the absence of frequent monitoring.
* Severe hepatic insufficiency.
## Adverse Effects
Common adverse effects include gastrointestinal disturbances (nausea, vomiting, diarrhea), rash, and urticaria. Serious adverse effects can include:
* **Hypersensitivity Reactions:** Stevens-Johnson syndrome, toxic epidermal necrolysis, exfoliative dermatitis, anaphylaxis.
* **Hematologic Effects:** Agranulocytosis, aplastic anemia, thrombocytopenia, leukopenia, megaloblastic anemia (especially with prolonged use or in folate-deficient states).
* **Renal Effects:** Crystalluria, interstitial nephritis.
* **Hepatic Effects:** Elevated liver enzymes, hepatitis, cholestatic jaundice.
* **Neurologic Effects:** Headache, dizziness, aseptic meningitis.
* **Electrolyte Disturbances:** Hyperkalemia.
## Key Drug Interactions
* **Warfarin:** Increased anticoagulant effect. Monitor INR closely.
* **Methotrexate:** Increased methotrexate toxicity.
* **Potassium-sparing Diuretics (e.g., spironolactone, amiloride):** Increased risk of hyperkalemia.
* **ACE Inhibitors/ARBs:** Increased risk of hyperkalemia.
* **Cyclosporine:** Increased cyclosporine levels, potentially leading to nephrotoxicity.
* **Oral Contraceptives:** May reduce efficacy.
## Monitoring
* **Renal function:** Essential, especially in patients with pre-existing renal disease or those receiving prolonged therapy.
* **Complete blood counts (CBCs):** Periodic monitoring is recommended, particularly for prolonged therapy or in immunocompromised patients.
* **Liver function tests:** If signs of hepatic dysfunction develop.
* **Electrolytes:** Especially potassium.
* **Therapeutic drug monitoring:** Not routinely required but may be considered in specific situations (e.g., severe infections, renal impairment).
## Clinical Pearls
* Advise patients to drink plenty of fluids to prevent crystalluria.
* Co-trimoxazole can cause photosensitivity; advise patients to use sun protection.
* Discontinue use immediately if a rash develops, as it may be an early sign of a severe reaction.
* Folic acid supplementation (e.g., 1 mg daily) may be considered for patients on long-term therapy to prevent hematologic side effects, especially in those at risk for folate deficiency.
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*This information is intended for healthcare professionals. Always consult the official prescribing information and relevant guidelines for complete details and to ensure patient safety. Drug information can change, and this response may not reflect the most current data.*
