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# Co-Trimoxazole (Trimethoprim/Sulfamethoxazole)
## Overview
Co-trimoxazole is a combination antibiotic consisting of trimethoprim and sulfamethoxazole in a fixed ratio of 1:5. It inhibits sequential steps in the folic acid synthesis pathway, leading to bacterial cell death. Available formulations include oral tablets, oral suspension, and intravenous injection.
## Primary Indications
* Urinary tract infections (UTIs)
* Pneumocystis jirovecii pneumonia (PJP) prophylaxis and treatment
* Acute exacerbations of chronic bronchitis
* Shigellosis
* Traveler's diarrhea (enterotoxigenic *E. coli*)
* *Staphylococcus aureus* infections (including MRSA in some skin and soft tissue infections, depending on susceptibility)
## Adult Dosing
* **Standard Dose:** 1 double-strength (DS) tablet (160 mg trimethoprim / 800 mg sulfamethoxazole) every 12 hours.
* **UTIs:** 1 DS tablet every 12 hours for 3-7 days.
* **PJP Treatment:** 15-20 mg/kg/day trimethoprim component, divided into 3-4 doses, for 14-21 days. This typically equates to 2 DS tablets every 6 hours.
* **PJP Prophylaxis:** 1 DS tablet once daily, or 1 DS tablet three times weekly (depending on local guidelines and risk factors).
* **Shigellosis:** 1 DS tablet every 12 hours for 5 days.
* **Traveler's Diarrhea:** 1 DS tablet every 12 hours for up to 5 days.
* **IV Dosing:** Generally 15-20 mg/kg/day trimethoprim component, divided into 3-4 doses. Specific infusion rate instructions are critical due to potential for hypersensitivity reactions.
Maximum dose is generally limited by the risk of toxicity, but in severe infections, doses may be higher under close monitoring.
## Pediatric Dosing
Dosing is based on the trimethoprim component, calculated per kilogram of body weight.
* **General Infections:** 8-10 mg/kg/day trimethoprim component, divided into two doses every 12 hours. For a 5 mL suspension containing 40 mg trimethoprim and 200 mg sulfamethoxazole per 5 mL:
* Example: A 20 kg child would receive 160-200 mg/day of trimethoprim, given as 80-100 mg per dose every 12 hours. This equates to approximately 10-12.5 mL of suspension per dose.
* **PJP Treatment:** 15-20 mg/kg/day trimethoprim component, divided into 3-4 doses every 6-8 hours.
* **PJP Prophylaxis:** 5 mg/kg/day trimethoprim component, divided into two doses every 12 hours, or 150 mg/m² body surface area/day divided into two doses, given daily or three times weekly.
*Note: Pediatric dosing can vary significantly based on indication and local protocols. Always consult specific guidelines.*
## Dose Adjustments
* **Renal Impairment:** Dose adjustment is necessary based on creatinine clearance (CrCl).
* CrCl > 30 mL/min: No adjustment needed.
* CrCl 15-30 mL/min: Give usual dose every 18 hours.
* CrCl < 15 mL/min: Use with caution; generally avoid or give half the usual dose every 24 hours, with close monitoring.
* **Hepatic Impairment:** Use with caution; no specific dose adjustment is typically outlined, but clinical monitoring is essential.
## Contraindications
* Known hypersensitivity to trimethoprim, sulfonamides, or any component of the formulation.
* History of drug-induced immune thrombocytopenia with sulfonamides or trimethoprim.
* Marked erythema multiforme (including Stevens-Johnson syndrome) or toxic epidermal necrolysis history.
* Megaloblastic anemia due to folate deficiency.
* Premature infants and full-term infants during the first 2 months of life (risk of kernicterus).
* Severe renal or hepatic insufficiency when practicable laboratory evaluation of serum levels cannot be obtained.
* Use in patients with a creatinine clearance below 15 mL/min.
## Adverse Effects
Common adverse effects include rash, pruritus, nausea, vomiting, and diarrhea. More serious effects include:
* **Hematologic:** Aplastic anemia, agranulocytosis, megaloblastic anemia (especially with prolonged use or in folate-deficient patients).
* **Dermatologic:** Stevens-Johnson syndrome (SJS), toxic epidermal necrolysis (TEN), exfoliative dermatitis, photosensitivity.
* **Hepatic:** Elevated liver enzymes, hepatitis, cholestatic jaundice.
* **Renal:** Interstitial nephritis, elevated creatinine.
* **Electrolyte:** Hyperkalemia.
* **Other:** Dizziness, headache, hallucinations, aseptic meningitis, hypoglycemia.
## Key Drug Interactions
* **Warfarin:** Co-trimoxazole can potentiate the anticoagulant effect of warfarin, increasing bleeding risk. Monitor INR closely.
* **ACE Inhibitors/ARBs:** Increased risk of hyperkalemia.
* **Potassium-Sparing Diuretics:** Increased risk of hyperkalemia.
* **Methotrexate:** Increased risk of bone marrow suppression, especially when given with leucovorin rescue.
* **Cyclosporine:** Increased cyclosporine levels, potentially leading to nephrotoxicity.
* **Digoxin:** Increased digoxin levels.
* **Oral Hypoglycemics:** May potentiate hypoglycemic effects.
* **Phenytoin:** Co-trimoxazole can inhibit phenytoin metabolism, leading to increased phenytoin levels.
* **Tricyclic Antidepressants (TCAs):** Limited data, but potential for increased toxicity.
* **Potassium Supplements:** Increased risk of hyperkalemia.
## Monitoring
* **Renal function:** Baseline and periodic monitoring of serum creatinine and BUN, especially in patients with pre-existing renal disease.
* **Hematologic:** Complete blood count (CBC) with differential before and during treatment, particularly for prolonged therapy or in high-risk patients.
* **Electrolytes:** Particularly potassium, especially in patients with renal impairment or those taking other potassium-altering medications.
* **Liver function tests:** Periodic monitoring may be warranted in some patients.
* **Clinical response:** Assess for improvement in signs and symptoms of infection.
* **Skin:** Monitor closely for signs of rash, which can be a precursor to severe reactions.
## Clinical Pearls
* Advise patients to drink plenty of fluids to prevent crystalluria.
* Emphasize the importance of completing the full course of therapy even if symptoms improve.
* Educate patients about potential adverse effects, especially rash and signs of hypersensitivity, and to report them immediately.
* Co-trimoxazole is teratogenic in the first trimester and can displace bilirubin in the term newborn; avoid in late pregnancy and infancy.
* Oral suspension contains phenylalanine and should be used with caution in patients with phenylketonuria.
* IV formulation should be infused slowly over 60-90 minutes to minimize infusion-related reactions.
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*Disclaimer: This information is intended for clinical use and does not substitute for professional medical judgment. Always consult the most current prescribing information and relevant clinical guidelines for definitive dosing, indications, contraindications, and safety information. Local formularies and protocols may influence specific recommendations.*
