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# Sulfamethoxazole/Trimethoprim (Co-trimoxazole)
## Overview
Co-trimoxazole is a combination antibiotic consisting of sulfamethoxazole (a sulfonamide) and trimethoprim (a dihydrofolate reductase inhibitor). The combination provides sequential blockade of folic acid synthesis, leading to a potent bactericidal effect.
## Primary Indications
* **Urinary Tract Infections (UTIs):** Acute uncomplicated UTIs, and prophylaxis for recurrent UTIs.
* **Pneumocystis jirovecii Pneumonia (PJP):** Treatment and prophylaxis.
* **Bacterial Infections:** Including otitis media, bronchitis, and shigellosis. Also used for susceptible strains of *Staphylococcus aureus*.
## Adult Dosing
* **UTIs:** 160 mg trimethoprim/800 mg sulfamethoxazole (1 double-strength tablet) every 12 hours for 3-7 days (uncomplicated) or 7-14 days (complicated).
* **PJP Prophylaxis:** 1 double-strength tablet daily or 1 single-strength tablet twice daily.
* **PJP Treatment:** 15 mg/kg/day trimethoprim component divided into 3 or 4 doses (equivalent to 15-20 mg/kg/day of TMP and 75-100 mg/kg/day of SMX) for 14-21 days.
* **General Infections:** Varies by indication and severity; typically 160 mg trimethoprim/800 mg sulfamethoxazole every 12 hours.
## Pediatric Dosing
Dosing is based on the trimethoprim component.
* **UTIs (uncomplicated):** 8 mg/kg/day trimethoprim, given in 2 divided doses every 12 hours, for 3-7 days.
* **PJP Prophylaxis:** 5 mg/kg/day trimethoprim, given in 2 divided doses every 12 hours, 3 times a week (e.g., Monday, Wednesday, Friday).
* **PJP Treatment:** 15 mg/kg/day trimethoprim, given in 4 divided doses every 6 hours for 14-21 days.
* **Other Infections:** 8-10 mg/kg/day trimethoprim, given in 2 divided doses every 12 hours.
**Note:** Pediatric dosing recommendations can vary; always consult current pediatric guidelines or protocols.
## Dose Adjustments
* **Renal Impairment (CrCl < 30 mL/min):**
* CrCl 15-30 mL/min: Administer 50% of the usual dose.
* CrCl < 15 mL/min: Contraindicated or administer 25% of the usual dose every 12 hours.
* Dosing frequency may also need adjustment.
## Contraindications
* Known hypersensitivity to trimethoprim, sulfonamides, or any component of the formulation.
* Infants < 2 months of age (risk of kernicterus).
* History of drug-induced thrombocytopenia with sulfonamides or trimethoprim.
* Marked anemia, megaloblastic anemia due to folate deficiency, or G6PD deficiency (sulfonamide moiety can precipitate hemolysis).
* Severe renal or hepatic insufficiency where frequent clinical and laboratory monitoring is not possible.
## Adverse Effects
* **Common:** Rash, nausea, vomiting, diarrhea, anorexia.
* **Serious:** Severe skin reactions (Stevens-Johnson syndrome, toxic epidermal necrolysis), hyperkalemia, bone marrow suppression (anemia, leukopenia, thrombocytopenia), hepatotoxicity, renal impairment, interstitial nephritis, photosensitivity, angioedema.
## Key Drug Interactions
* **Warfarin:** Co-trimoxazole can inhibit the metabolism of warfarin, leading to increased INR and risk of bleeding. Monitor INR closely.
* **ACE Inhibitors/ARBs/Potassium-Sparing Diuretics:** Increased risk of hyperkalemia.
* **Methotrexate:** Co-trimoxazole can displace methotrexate from plasma protein binding and inhibit its metabolism, increasing methotrexate toxicity.
* **Cyclosporine:** Increased risk of nephrotoxicity.
* **Potassium Supplements/Potassium-Containing Salt Substitutes:** Increased risk of hyperkalemia.
* **Procainamide and Amantadine:** May increase serum concentrations.
* **Sulfonylureas:** Increased risk of hypoglycemia.
## Monitoring
* **Renal function:** Baseline and during therapy, especially in patients with impaired renal function.
* **Electrolytes:** Particularly potassium, especially in patients with renal impairment or taking other drugs that affect potassium levels.
* **Complete blood count (CBC):** Baseline and periodically during prolonged therapy, or if signs of infection or bleeding occur.
* **Liver function tests:** Baseline and periodically, especially in patients with hepatic impairment or during prolonged therapy.
* **Clinical signs of infection and hypersensitivity reactions.**
## Clinical Pearls
* Ensure adequate hydration to prevent crystalluria.
* Advise patients to report any rash, fever, sore throat, or unusual bleeding/bruising immediately.
* Co-trimoxazole can interfere with urine glucose and protein tests.
* For PJP prophylaxis in immunocompromised patients, consistent adherence is crucial.
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**Disclaimer:** This information is intended for clinical use and does not replace a comprehensive review of the drug's prescribing information. Always consult the most current product labeling and institutional protocols for definitive dosing, administration, and safety guidance.
