What is the typical dosage (sig) of ampicillin (ampicillin) for an uncomplicated urinary tract infection (UTI)?

Ampicillin Dosing for Urinary Tract Infections

Ampicillin is NOT a first-line agent for uncomplicated UTIs and should only be used in combination with an aminoglycoside for hospitalized patients with pyelonephritis or complicated UTIs when susceptibility is confirmed. 1

Key Dosing Information

For Uncomplicated Lower UTI (Cystitis)

• Ampicillin is not recommended as first-line therapy for uncomplicated cystitis due to high resistance rates among E. coli 2
• If used based on culture susceptibility: 500 mg orally four times daily (q.i.d.) 3
• Administer at least 30 minutes before or 2 hours after meals for optimal absorption 3
• Duration: Continue for 48-72 hours after symptoms resolve, with a minimum of 7 days total for genitourinary infections 3

For Pyelonephritis Requiring Hospitalization

• Ampicillin should only be used in combination with an aminoglycoside (such as gentamicin 5 mg/kg IV once daily), never as monotherapy 1
• This combination is reserved for hospitalized patients with uncomplicated pyelonephritis when local resistance patterns support its use 1
• The 2024 European Association of Urology guidelines list aminoglycosides "with or without ampicillin" as an option, but fluoroquinolones, cephalosporins, and piperacillin/tazobactam are preferred first-line parenteral agents 1

For Complicated UTIs

• Ampicillin monotherapy is inadequate for complicated UTIs 4
• If susceptibility testing confirms sensitivity, use 500 mg orally q.i.d. for genitourinary infections; severe or chronic infections may require larger doses 3
• Duration: 7-14 days (14 days for men when prostatitis cannot be excluded) 1, 4
• Frequent bacteriologic and clinical appraisal is necessary during and after therapy for chronic infections 3

Critical Clinical Considerations

Why Ampicillin Is Rarely Used

• High resistance rates among E. coli (the most common uropathogen) make ampicillin unreliable for empiric therapy 2
• The 2024 EAU guidelines do not include ampicillin in any oral regimen tables for uncomplicated cystitis or pyelonephritis 1
• First-line agents for uncomplicated cystitis include nitrofurantoin, fosfomycin, and pivmecillinam 1
• First-line agents for pyelonephritis include fluoroquinolones (if local resistance <10%) and oral cephalosporins 1

When Ampicillin Might Be Considered

• Only after culture and susceptibility testing confirms susceptibility 3, 2
• In settings where the organism is known to be susceptible (e.g., Enterococcus species in some cases)
• As part of combination therapy with aminoglycosides for hospitalized patients with pyelonephritis 1

Important Warnings

• Do not use smaller doses than recommended 3
• For streptococcal infections, treat for minimum 10 days to prevent rheumatic fever or glomerulonephritis 3
• Obtain follow-up cultures 7-14 days after therapy to confirm eradication 3
• In women treated for gonorrhea (not typical UTI), also obtain cultures from endocervical and anal canals 3

Pediatric Dosing

• For children ≤20 kg with genitourinary infections: 100 mg/kg/day divided q.i.d. 3
• Do not exceed adult doses 3

Bottom Line Algorithm

1. Is this uncomplicated cystitis? → Use nitrofurantoin, fosfomycin, or pivmecillinam instead 1
2. Is this uncomplicated pyelonephritis (outpatient)? → Use fluoroquinolones or oral cephalosporins instead 1
3. Is this pyelonephritis requiring hospitalization? → Use IV fluoroquinolone, cephalosporin, or aminoglycoside ± ampicillin 1
4. Is this complicated UTI with confirmed ampicillin susceptibility? → Consider 500 mg PO q.i.d. for 7-14 days 3, 4
5. Is resistance unknown? → Do not use ampicillin empirically 2