What is the recommended dosage and treatment duration for ampicillin in treating bacterial infections?

Ampicillin Dosing and Treatment Duration for Bacterial Infections

For most bacterial infections, ampicillin dosing ranges from 250-500 mg every 6 hours for mild infections to 150-200 mg/kg/day (up to 12 g/day) divided every 3-4 hours for severe infections like meningitis and endocarditis, with treatment duration of 48-72 hours beyond symptom resolution for most infections, 10 days minimum for streptococcal infections, and 4-6 weeks for endocarditis. 1, 2

Adult Dosing by Infection Type

Respiratory Tract and Soft Tissue Infections

• Standard dose: 250-500 mg IV/PO every 6 hours for patients ≥40 kg 1
• Oral administration should occur at least 30 minutes before or 2 hours after meals for maximal absorption 3

Genitourinary and Gastrointestinal Tract Infections

• Standard dose: 500 mg IV/PO every 6 hours for patients ≥40 kg 1
• For uncomplicated UTIs caused by VRE: High-dose ampicillin 18-30 g/day IV in divided doses achieves clinical cure rates of 88.1% and microbiological eradication of 86% 4, 2
• Alternative oral option: Amoxicillin 500 mg PO/IV every 8 hours 4, 2

Gonorrhea

• Single-dose regimen: Two doses of 500 mg each at 8-12 hour intervals 1
• Alternative oral regimen: 3.5 g ampicillin as single dose with 1 g probenecid simultaneously 3
• For complications (prostatitis, epididymitis), prolonged intensive therapy is required 1, 3

Infective Endocarditis

• Enterococcal endocarditis: 200-300 mg/kg/day IV divided every 4-6 hours (up to 12 g/day) for 4-6 weeks 2, 4
• Combination therapy: Add gentamicin 3 mg/kg/day IV/IM in 1 dose for first 2 weeks (some experts recommend only 2 weeks of gentamicin) 4, 2
• This combination is not active against E. faecium 4

HACEK Group Infections

• If beta-lactamase negative: Ampicillin 12 g/day IV in 4-6 doses plus gentamicin 3 mg/kg/day for 4-6 weeks 4, 2
• Critical caveat: Ceftriaxone is now preferred first-line due to increasing beta-lactamase production; ampicillin should not be used without susceptibility testing 2

Bacterial Meningitis and Septicemia

• Adults and children: 150-200 mg/kg/day IV in equally divided doses every 3-4 hours 1
• Initiate with IV drip therapy, may continue with IM injections 1
• Research shows 150 mg/kg/day is as effective as 400 mg/kg/day for meningitis, with equivalent outcomes 5

Pediatric Dosing

Standard Infections (Children >20 kg)

• Respiratory tract: 250 mg every 6 hours 1
• GI/GU tract: 500 mg every 6 hours 1

Standard Infections (Children <40 kg)

• Respiratory/soft tissue: 25-50 mg/kg/day divided every 6-8 hours 1
• GI/GU tract: 50 mg/kg/day divided every 6-8 hours 1

Bacterial Meningitis and Septicemia (Neonates ≤28 days)

• Gestational age ≤34 weeks, postnatal age ≤7 days: 100 mg/kg/day divided every 12 hours 1, 2
• Gestational age ≤34 weeks, postnatal age 8-27 days: 150 mg/kg/day divided every 12 hours 1, 2
• Gestational age >34 weeks, postnatal age ≤28 days: 150 mg/kg/day divided every 8 hours 1, 2
• Critical warning: Do not use Bacteriostatic Water for Injection as diluent in newborns 1

Treatment Duration Guidelines

General Principles

• Minimum duration: Continue 48-72 hours beyond symptom resolution or bacterial eradication 1, 3
• Streptococcal infections: Minimum 10 days to prevent acute rheumatic fever and glomerulonephritis 1, 3, 2

Specific Infections

• Uncomplicated UTI: 3-7 days 2
• Infective endocarditis: 4-6 weeks 2, 4
• Respiratory tract infections: Standard 5-7 days, though research in children shows 3 days may be noninferior to 7 days for community-acquired pneumonia 6
• Chronic/stubborn infections: May require several weeks with bacteriological follow-up for several months 1, 3

Administration and Stability Considerations

Intramuscular Administration

• Reconstitute with Sterile or Bacteriostatic Water for Injection per manufacturer guidelines 1
• 250 mg vial: Add 1 mL for 250 mg/mL concentration 1
• 500 mg vial: Add 1.8 mL for 250 mg/mL concentration 1
• 1 g vial: Add 3.5 mL for 250 mg/mL concentration 1

Intravenous Administration

• Direct IV: Add 5 mL Sterile Water, administer slowly over 3-5 minutes 1
• Critical stability warning: Use only freshly prepared solutions; administer within 1 hour as potency decreases significantly after this period 1
• Continuous infusion via ambulatory pump is feasible for outpatient endocarditis treatment, with enhanced stability >24 hours at room temperature 7

Special Populations and Resistance Considerations

Vancomycin-Resistant Enterococci (VRE)

• High-dose ampicillin (18-30 g/day IV) can overcome high MICs in uncomplicated UTIs due to high urinary concentrations 4
• This approach achieves 88.1% clinical cure despite ampicillin resistance 4
• Important distinction: Ampicillin remains drug of choice for enterococcal UTIs regardless of susceptibility testing 4

Beta-Lactamase Producing Organisms

• If beta-lactamase production confirmed, replace ampicillin with ampicillin-sulbactam or amoxicillin-clavulanate 4
• Ampicillin/sulbactam in 2:1 ratio is effective for severe infections including respiratory, gynecological, intra-abdominal, and diabetic foot infections 8

Multiresistant Enterococci

• If resistant to ampicillin, beta-lactams, and vancomycin, consider daptomycin 10 mg/kg/day plus ampicillin 200 mg/kg/day IV, or linezolid 600 mg every 12 hours for ≥8 weeks 4

Common Pitfalls to Avoid

• Never use doses smaller than recommended even for mild infections, as this promotes resistance 1, 3
• Do not extend treatment beyond necessary duration without clear indication, particularly beyond 7 days for uncomplicated UTI 2
• Always obtain cultures before initiating therapy when feasible, especially for VRE where susceptibility varies 2
• Monitor for syphilis in gonorrhea cases: Perform darkfield examination if primary lesion suspected; otherwise monthly serological tests for minimum 4 months 1, 3
• Ensure adequate follow-up: Bacteriological and clinical appraisal needed during and after therapy for chronic infections 1, 3
• Administer oral ampicillin on empty stomach (30 minutes before or 2 hours after meals) for optimal absorption 3