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# Co-Trimoxazole (Trimethoprim/Sulfamethoxazole)
## Overview
Co-trimoxazole is a combination antibiotic consisting of trimethoprim and sulfamethoxazole in a 1:5 ratio. It inhibits sequential steps in bacterial folic acid synthesis.
## Primary Indications
* **Urinary Tract Infections (UTIs):** Uncomplicated UTIs.
* **Pneumocystis jirovecii pneumonia (PCP):** Treatment and prophylaxis.
* **Shigellosis.**
* **Acute exacerbations of chronic bronchitis.**
* **Nocardiosis.**
* **Melioidosis.**
* **Toxoplasmosis** (in combination with pyrimethamine).
## Adult Dosing
* **Uncomplicated UTIs:** 1 double-strength (DS) tablet (800 mg sulfamethoxazole/160 mg trimethoprim) orally every 12 hours for 3-7 days.
* **PCP Treatment:** 15 mg/kg/day trimethoprim component divided into 4 doses (e.g., 2 DS tablets every 6 hours) for 14-21 days.
* **PCP Prophylaxis (HIV-infected patients):** 1 DS tablet orally once daily, or 1 DS tablet 3 times a week (depending on local guidelines and risk factors).
* **Shigellosis:** 160 mg trimethoprim/800 mg sulfamethoxazole (1 DS tablet) orally every 12 hours for 5 days.
* **Other indications:** Dosing varies by indication; consult specific guidelines. A common regimen for many infections is 1 DS tablet every 12 hours, with higher doses (e.g., 2 DS tablets every 12 hours) used for more severe infections.
## Pediatric Dosing
Dosing is based on the trimethoprim component.
* **UTIs/Shigellosis:** 8 mg/kg/day trimethoprim and 40 mg/kg/day sulfamethoxazole, divided into 2 doses every 12 hours. Maximum daily dose is typically 320 mg trimethoprim.
* **PCP Prophylaxis:** 5 mg/kg/day trimethoprim and 25 mg/kg/day sulfamethoxazole, divided into 2 doses daily.
* **PCP Treatment:** 15 mg/kg/day trimethoprim and 75 mg/kg/day sulfamethoxazole, divided into 4 doses every 6 hours.
* **Note:** Due to potential for kernicterus, co-trimoxazole is generally avoided in infants younger than 2 months.
## Dose Adjustments
* **Renal Impairment:**
* Creatinine clearance (CrCl) > 30 mL/min: Full dose.
* CrCl 15-30 mL/min: Half the usual dose.
* CrCl < 15 mL/min: Avoid use or use with extreme caution at significantly reduced doses (consult specific guidelines).
## Contraindications
* Known hypersensitivity to trimethoprim, sulfonamides, or any component of the formulation.
* Documented megaloblastic anemia due to folate deficiency.
* Infants younger than 2 months (risk of kernicterus).
* Severe renal or hepatic insufficiency where drug levels cannot be monitored.
* History of drug-induced thrombocytopenia with sulfonamides or trimethoprim.
## Adverse Effects
Common: Rash, nausea, vomiting, diarrhea, hyperkalemia.
Serious: Stevens-Johnson syndrome, toxic epidermal necrolysis, exfoliative dermatitis, severe allergic reactions, photosensitivity, bone marrow suppression (anemia, leukopenia, thrombocytopenia), hepatitis, renal failure, hyperkalemia, hypoglycemia.
## Key Drug Interactions
* **Warfarin:** Increased anticoagulant effect. Monitor INR closely.
* **ACE Inhibitors/ARBs/Potassium-Sparing Diuretics:** Increased risk of hyperkalemia.
* **Methotrexate:** Increased methotrexate toxicity (bone marrow suppression, renal toxicity) due to displacement from plasma protein binding.
* **Cyclosporine:** Increased cyclosporine levels, potentially leading to nephrotoxicity.
* **Digoxin:** Increased digoxin levels, especially in patients with renal impairment or AIDS.
* **Potassium Supplements:** Increased risk of hyperkalemia.
* **Oral Hypoglycemics:** Potentiated hypoglycemia.
* **Phenytoin:** Increased phenytoin levels.
* **Rifampin:** May decrease co-trimoxazole levels.
## Monitoring
* **Renal function:** Baseline and periodically, especially in patients with renal impairment.
* **Electrolytes:** Particularly potassium, especially in patients with renal impairment or those taking other drugs that affect potassium.
* **Complete blood count (CBC) with differential:** Baseline and periodically, especially with prolonged therapy or in immunocompromised patients. Monitor for signs of hematologic toxicity (pallor, fatigue, fever, bleeding, bruising).
* **Liver function tests:** Baseline and periodically, especially in patients with hepatic impairment.
* **Therapeutic drug monitoring:** May be considered in specific situations (e.g., severe infections, renal impairment, AIDS patients with PCP treatment) to maintain trimethoprim levels between 4-8 mcg/mL.
## Clinical Pearls
* Encourage adequate fluid intake to prevent crystalluria.
* Advise patients to report any rash, sore throat, fever, or unusual bleeding/bruising immediately.
* Photosensitivity can occur; advise sun protection.
* Sulfonamides can displace bilirubin from albumin, posing a risk to newborns. Avoid in late pregnancy.
* Co-trimoxazole is an FDA-approved agent for PCP prophylaxis in HIV-infected individuals.
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*Disclaimer: This information is intended for clinical professionals and does not substitute for comprehensive drug information resources. Always consult the current prescribing information and relevant clinical guidelines before making treatment decisions.*
