What is the appropriate Bactrim (trimethoprim‑sulfamethoxazole) dose for an uncomplicated urinary tract infection in a healthy adult and how should it be adjusted for renal impairment, pregnancy, children, or sulfonamide allergy?

Bactrim Dosing for Urinary Tract Infection

Standard Adult Dosing for Uncomplicated Cystitis

For women with uncomplicated cystitis, prescribe Bactrim DS (trimethoprim 160 mg/sulfamethoxazole 800 mg) one double-strength tablet twice daily for 3 days, provided local E. coli resistance is below 20%. 1

• This 3-day regimen achieves 90-100% clinical cure rates and 91-100% bacterial eradication when organisms are susceptible 2, 1
• For men with uncomplicated UTI, extend treatment to 7 days using the same twice-daily dosing, as the 3-day regimen studied in women is inadequate for males 1, 3

Critical Resistance Threshold

Do not use Bactrim empirically when local E. coli resistance exceeds 20%, as treatment failures outweigh benefits at this threshold 1

• When organisms are resistant, clinical cure rates plummet to only 41-54%, making treatment failure the expected outcome 1
• Avoid Bactrim in patients who used it within the preceding 3-6 months, as recent exposure independently predicts resistance 1
• Avoid empiric use in patients with recent international travel (within 3-6 months), which is associated with higher resistance rates 1, 3

Renal Dose Adjustments

For creatinine clearance >30 mL/min: use standard dose of one double-strength tablet twice daily 3

For creatinine clearance 15-30 mL/min: reduce to half-dose (one single-strength tablet or half of double-strength) 3

For creatinine clearance <15 mL/min: use half-dose or select an alternative agent 3

• Monitor serum creatinine and electrolytes 2-3 times weekly during therapy in patients with renal impairment 3
• Trimethoprim can cause hyperkalemia; check baseline and monitor potassium levels regularly 3
• Ensure adequate hydration (at least 1.5 liters daily) to prevent crystalluria 3

Pyelonephritis Dosing

For acute uncomplicated pyelonephritis, prescribe Bactrim DS twice daily for 14 days, but only after confirming susceptibility 2, 1

• If using Bactrim empirically when susceptibility is unknown, administer an initial intravenous dose of ceftriaxone 1 g or a consolidated 24-hour aminoglycoside dose first 2
• Fluoroquinolones (ciprofloxacin 500 mg twice daily for 7 days or levofloxacin 750 mg daily for 5 days) are preferred over Bactrim for pyelonephritis when local resistance is <10% 2, 1

Pregnancy Considerations

Avoid Bactrim in the last trimester of pregnancy due to potential fetal risks 1, 3

• Trimethoprim alone should not be used in the first trimester 3
• Alternative first-line agents for pregnant women include nitrofurantoin (avoid near term) or cephalosporins 3

Pediatric Dosing

Single-dose trimethoprim regimens in children clear bacteriuria effectively but carry a 23% risk of asymptomatic recurrence within 10 days, compared to 2% with a 7-day course 4

• For children, a 7-day course of co-trimoxazole is more effective than single-dose therapy at preventing early recurrence 4

Sulfonamide Allergy

If the patient has a documented sulfonamide allergy, Bactrim is absolutely contraindicated

Alternative first-line agents include:

• Nitrofurantoin monohydrate/macrocrystals 100 mg twice daily for 5 days (90% clinical cure, 92% bacterial cure) 1
• Fosfomycin trometamol 3 g single dose (convenient single-dose therapy with minimal resistance) 1, 3
• Ciprofloxacin 250 mg twice daily for 3 days (93-97% eradication rates, but reserve for cases where other agents cannot be used) 1

Common Pitfalls to Avoid

• Do not prescribe the 3-day regimen for men—this is treatment failure waiting to happen; men require 7 days minimum 1, 3
• Do not rely on hospital antibiograms for community-acquired cystitis, as they overestimate resistance by reflecting complicated infections; use outpatient surveillance data instead 1
• Do not fail to adjust dose in renal impairment (CrCl <30 mL/min)—this significantly increases toxicity risk, particularly hyperkalemia and bone marrow suppression 3
• Do not combine Bactrim with ciprofloxacin for uncomplicated UTI; monotherapy with a single effective agent is the standard of care 3
• Common adverse effects include rash, urticaria, nausea, vomiting, and hematologic abnormalities (thrombocytopenia, neutropenia) 1, 3

When Bactrim Is Not Appropriate

Select an alternative agent when:

• Local E. coli resistance ≥20% 1
• Recent Bactrim use within 3-6 months 1
• Recent international travel within 3-6 months 1
• Pregnancy (especially third trimester) 1, 3
• Sulfonamide allergy 1
• Marked hepatic damage 1
• Creatinine clearance <15 mL/min without dose adjustment 3