Ampicillin: Indications, Dosing, Contraindications, and Adverse Effects
Ampicillin is a broad-spectrum penicillin antibiotic indicated for susceptible bacterial infections including meningitis, sepsis, pneumonia, urinary tract infections, and endocarditis, with dosing highly dependent on patient age, infection site, and severity.
Indications
Ampicillin covers penicillin-sensitive organisms and certain gram-negative species including E. coli, Proteus mirabilis, Salmonella, Shigella, and Haemophilus influenzae 1. Key clinical indications include:
Neonatal and Infant Infections (Birth to 60 Days)
- Bacterial meningitis: Ampicillin is essential empiric coverage for Listeria monocytogenes and Group B Streptococcus in neonates and young infants 2
- Sepsis without identified focus: Combined with gentamicin or ceftazidime for empiric coverage 2
- Urinary tract infections: Particularly in infants 8-21 days old, combined with gentamicin or ceftazidime 2
Pediatric Infections (Beyond 60 Days)
- Community-acquired pneumonia: For Streptococcus pneumoniae with penicillin MICs ≤2.0 μg/mL and Group A Streptococcus 3
- Catheter-related infections: As part of empiric therapy 4
Adult Infections
- Enterococcal endocarditis: Particularly for ampicillin-susceptible strains 5
- HACEK endocarditis: Though ceftriaxone is now preferred due to β-lactamase-producing strains 5
Important caveat: Ampicillin should NOT be used for penicillinase-producing organisms 6. For H. influenzae, verify β-lactamase status—if positive, switch to ceftriaxone or amoxicillin-clavulanate 3.
Dosing Regimens
Neonatal Dosing (IV/IM)
For meningitis 2:
- 8-21 days old: 300 mg/kg/day divided every 6 hours
- 22-28 days old: 300 mg/kg/day divided every 6 hours
- 29-60 days old: Continue 300 mg/kg/day divided every 6 hours (though ceftriaxone becomes preferred)
For sepsis/UTI 2:
- 8-21 days old: 150 mg/kg/day divided every 8 hours
- 22-28 days old: 150 mg/kg/day divided every 8 hours
- 29-60 days old: Transition to ceftriaxone preferred
Weight-based neonatal dosing 4:
- ≤7 days old and ≤2000 g: 50 mg/kg/day divided every 12 hours
- ≤7 days old and >2000 g: 75 mg/kg/day divided every 8 hours
- >7 days old and <1200 g: 50 mg/kg/day divided every 12 hours
- >7 days old and 1200-2000 g: 75 mg/kg/day divided every 8 hours
- >7 days old and >2000 g: 100 mg/kg/day divided every 6 hours
Pediatric Dosing (Beyond Neonatal Period)
Standard infections 4:
- Infants and children: 100-200 mg/kg/day divided every 6 hours IV
- 150-200 mg/kg/day divided every 6 hours for S. pneumoniae with penicillin MICs ≤2.0 μg/mL
- 200 mg/kg/day divided every 6 hours for Group A Streptococcus
- 50 mg/kg/day divided every 6 hours for mild infections or step-down therapy (oral)
Meningitis 7:
- 50 mg/kg every 6 hours (QDS) for at least 10 days, though higher doses (300-400 mg/kg/day) may be used for resistant organisms 3
Adult Dosing
Enterococcal endocarditis 5:
- 12 g/24 hours IV in 6 equally divided doses (2 g every 4 hours) for ampicillin-susceptible strains
- Duration: 4-6 weeks depending on valve type
General severe infections 8:
- Typical dosing every 4-6 hours depending on renal function and infection severity
- Note: Ampicillin has a short half-life and is unstable at body temperature, requiring frequent dosing 8
Outpatient Parenteral Antimicrobial Therapy (OPAT)
Recent data supports continuous infusion models for OPAT 9:
- Ampicillin can be refrigerated for up to 72 hours, then continuously infused at room temperature over 24 hours with <10% loss of potency
- This allows twice-weekly OPAT delivery rather than every-4-6-hour dosing
Contraindications
Absolute contraindications 6:
- History of hypersensitivity to any penicillin: This is non-negotiable
- Infections caused by penicillinase-producing organisms: Ampicillin will fail
Relative considerations:
- Mononucleosis: High risk of non-allergic rash (though not a true contraindication, avoid if possible)
- Severe renal impairment: Requires dose adjustment
Adverse Effects
Common Adverse Effects
- Diarrhea: 7.74% incidence 10, more common than with newer aminopenicillins 11
- Nausea/vomiting: 7.74% 10
- Anorexia: 5.46% 10
- Pseudomembranous colitis: Rare but serious; ampicillin shows 0.7% incidence compared to 1.9% with clindamycin 12
- Maculopapular rash: 2.9% incidence 10, typically appears after first week of therapy
- Erythematous, mildly pruritic rash: Usually covers entire body including palms, soles, and oral mucosa; resolves in 3-7 days 6
Serious Adverse Effects
Hypersensitivity reactions 6:
- Anaphylaxis: Most serious reaction, usually with parenteral administration
- Urticaria, erythema multiforme, exfoliative dermatitis: Require immediate discontinuation
- Management: Antihistamines for mild reactions; systemic corticosteroids if necessary; emergency measures for anaphylaxis
Hematologic 6:
- Anemia, thrombocytopenia, thrombocytopenic purpura, eosinophilia, leukopenia, agranulocytosis
- Usually reversible upon discontinuation
Hepatic 6:
- Moderate elevation in SGOT (significance unknown)
Other 6:
- Laryngeal stridor, high fever (rare)
Gender and Route Considerations
- Females more susceptible to ampicillin adverse reactions 10
- IM route produces fewer adverse reactions than IV 10
Key Clinical Pitfalls
Do not substitute ampicillin for penicillin G when penicillin G would suffice—penicillin G is less expensive, often more active, and causes fewer rashes 1
Verify β-lactamase status for H. influenzae—58-82% of isolates remain susceptible, but β-lactamase producers require alternative therapy 13
HACEK organisms should be considered ampicillin-resistant—use ceftriaxone instead due to increasing β-lactamase production 5
Frequent dosing is required—ampicillin's short half-life necessitates every 4-6 hour dosing for most infections, though continuous infusion models are emerging for OPAT 8, 9
Monitor renal function—dose adjustment required in renal impairment
Distinguish colonization from infection—particularly relevant in ICU settings where ampicillin-resistant organisms may colonize without causing invasive disease