Which infections can be treated with cotrimoxazole (trimethoprim‑sulfamethoxazole)?

Cotrimoxazole (Trimethoprim-Sulfamethoxazole) Indications

Cotrimoxazole is FDA-approved and guideline-recommended for urinary tract infections, acute otitis media, acute exacerbations of chronic bronchitis, shigellosis, Pneumocystis jirovecii pneumonia (treatment and prophylaxis), and traveler's diarrhea, with additional evidence supporting its use in MRSA infections, nocardiosis, toxoplasmosis, and several other specific bacterial and protozoal infections. 1

FDA-Approved Indications

Urinary Tract Infections

• Cotrimoxazole is approved for UTIs caused by susceptible strains of Escherichia coli, Klebsiella species, Enterobacter species, Morganella morganii, Proteus mirabilis, and Proteus vulgaris. 1
• For acute uncomplicated cystitis in women, cotrimoxazole 160/800 mg (one double-strength tablet) twice daily for 3 days is appropriate when local resistance rates do not exceed 20% or when the infecting strain is known to be susceptible. 2
• Critical caveat: The 20% resistance threshold is based on expert opinion—if your local E. coli resistance to TMP-SMX exceeds 20%, choose nitrofurantoin or another agent instead. 2

Respiratory Tract Infections

• Acute otitis media in pediatric patients (≥2 months old) caused by susceptible Streptococcus pneumoniae or Haemophilus influenzae when cotrimoxazole offers advantage over other agents. 1
• Acute exacerbations of chronic bronchitis in adults due to susceptible S. pneumoniae or H. influenzae when deemed advantageous by the physician. 1
• Cotrimoxazole is not indicated for prophylactic or prolonged use in otitis media at any age. 1

Gastrointestinal Infections

• Shigellosis caused by susceptible Shigella flexneri and Shigella sonnei when antibacterial therapy is indicated. 1
• Traveler's diarrhea in adults due to susceptible enterotoxigenic E. coli. 1

Pneumocystis jirovecii Pneumonia (PJP)

• Treatment of documented PJP and prophylaxis in immunosuppressed individuals at increased risk. 1
• For prophylaxis in HIV-infected adults with CD4+ counts <200 cells/µL, the preferred regimen is one double-strength tablet (160/800 mg) daily, or one single-strength tablet daily, or one double-strength tablet three times weekly. 3
• Never coadminister cotrimoxazole with leucovorin during PJP treatment—a randomized trial in HIV patients showed treatment failure and excess mortality with this combination. 1
• Lower-dose regimens (<15 mg/kg/day of trimethoprim) show similar mortality but significantly fewer adverse reactions compared to conventional dosing (15-20 mg/kg/day). 4

Evidence-Based "Unconventional" Indications

MRSA Infections

• For MRSA skin and soft tissue infections, cotrimoxazole 320-640 mg TMP/1600-3200 mg SMZ total daily (one to two double-strength tablets twice daily) is recommended. 5
• For documented MRSA pneumonia, 5 mg/kg/dose IV every 8-12 hours is an alternative agent. 5
• Multiple retrospective and prospective studies demonstrate good clinical outcomes with cotrimoxazole for invasive MRSA infections. 6

Specific Bacterial Pathogens

• Nocardiosis: Cotrimoxazole is effective and considered a treatment of choice. 6, 7
• Brucellosis: Cotrimoxazole shows efficacy and is superior to trimethoprim alone. 7
• Listeria monocytogenes: Effective against susceptible strains. 6
• Stenotrophomonas maltophilia: Cotrimoxazole remains the drug of choice for this multidrug-resistant pathogen. 6
• Burkholderia pseudomallei (melioidosis): Weight-based dosing is recommended—160/800 mg every 12 hours for patients <40 kg, 240/1200 mg every 12 hours for 40-60 kg, and 320/1600 mg every 12 hours for >60 kg, with folic acid supplementation. 5, 8
• Burkholderia mallei (glanders): Effective for post-exposure prophylaxis. 8
• Coxiella burnetii (Q fever) and Tropheryma whipplei (Whipple's disease): Cotrimoxazole shows activity. 6

Protozoal Infections

• Toxoplasmosis: Cotrimoxazole is superior to trimethoprim alone and provides dual protection when used for PJP prophylaxis in HIV patients. 3, 7
• Cyclospora and Isospora infections: Cotrimoxazole is effective. 6
• Susceptible Plasmodium falciparum malaria: Shows activity in susceptible strains. 6

Other Pathogens

• Chancroid: Cotrimoxazole is superior to trimethoprim alone. 7
• Atypical mycobacteria: Some activity reported. 6

Critical Dosing Considerations

Pediatric Dosing

• For children >2 months: 8-12 mg/kg/day TMP and 40-60 mg/kg/day SMZ, divided every 12 hours. 5
• For pediatric PCP prophylaxis: 150 mg/m² TMP and 750 mg/m² SMZ daily, divided twice daily, given 3 consecutive days per week. 5

Renal Impairment

• Creatinine clearance significantly affects both TMP and SMZ pharmacokinetics—dose adjustment is mandatory in renal dysfunction. 9
• For patients on hemodialysis requiring prophylaxis: 500 mg three times weekly after dialysis. 5
• For CrCl >30 mL/min: standard three-times-weekly regimen is appropriate. 5
• Recent pharmacokinetic modeling suggests 50 mg/kg/day in three divided doses is feasible for CrCl <15 mL/min, while the guideline-recommended 90 mg/kg/day poses supratherapeutic exposure risk in patients with normal renal function. 9

Absolute Contraindications and Critical Warnings

Do Not Use For

• Group A β-hemolytic streptococcal infections—cotrimoxazole will not eradicate the organism and will not prevent rheumatic fever. 1
• Pregnancy at term—associated with increased risk of congenital malformations including neural tube defects, cardiovascular malformations, urinary tract defects, oral clefts, and club foot. 1
• G6PD deficiency—risk of hemolytic anemia. 5

Severe Adverse Reactions Requiring Immediate Discontinuation

• Discontinue at the first appearance of skin rash—may progress to Stevens-Johnson syndrome, toxic epidermal necrolysis, DRESS, or other severe cutaneous adverse reactions. 1
• Fatalities have occurred from fulminant hepatic necrosis, agranulocytosis, aplastic anemia, and other blood dyscrasias. 1
• Severe pulmonary reactions (respiratory failure requiring mechanical ventilation or ECMO) can occur within days to weeks of initiation. 1
• Circulatory shock with fever, severe hypotension, and confusion can occur within minutes to hours of re-challenge. 1

Hematologic Monitoring

• Obtain baseline complete blood count and platelet count before initiating therapy. 3
• Monitor CBC with platelets every 3-7 days during the first 2 weeks, then weekly during therapy and for 2 weeks after discontinuation. 3
• Never rechallenge a patient after documented drug-induced thrombocytopenia—recurrence is highly likely and potentially fatal. 3
• Patients with renal dysfunction, liver disease, malnutrition, or those receiving high doses are at particular risk for hematologic toxicity. 1

Electrolyte Abnormalities

• Hyperkalemia: High-dose TMP (as used in PJP) induces progressive but reversible hyperkalemia—close monitoring is warranted, especially in patients with renal insufficiency or those on drugs that induce hyperkalemia. 1
• Hyponatremia: Severe and symptomatic hyponatremia can occur, particularly in PJP treatment—evaluation and correction are necessary to prevent life-threatening complications. 1

Drug Interactions

• Avoid concurrent use with methotrexate at treatment doses—risk of severe bone marrow suppression. 5, 1
• Avoid concurrent thiazide diuretics in elderly patients—increased incidence of thrombocytopenia with purpura. 1
• Monitor prothrombin time and INR with warfarin—cotrimoxazole may prolong PT. 1
• Monitor serum phenytoin levels—cotrimoxazole inhibits hepatic metabolism of phenytoin. 1

Special Population Considerations

• AIDS patients have significantly higher incidence of adverse reactions (rash, fever, leukopenia, elevated transaminases) compared to non-AIDS patients. 1
• Hypoglycemia is a rare but potentially life-threatening adverse effect due to sulfonylurea-like effect, particularly in patients with renal insufficiency—refractory cases may require octreotide. 10

Resistance Considerations

• Cotrimoxazole maintains activity against major pathogens with still relatively low resistance rates for S. pneumoniae and H. influenzae. 11
• Resistance >10% is documented in E. coli, Shigella spp., Klebsiella pneumoniae, Providencia rettgeri, Morganella morganii, and methicillin-resistant staphylococci. 11
• In the era of widespread antibiotic resistance, cotrimoxazole represents a valuable option when used appropriately for susceptible organisms. 6