What are the recommended dosing, administration instructions, contraindications, and common adverse effects of ampicillin tablets for adults and children?

Ampicillin Tablets: Dosing, Administration, Contraindications, and Adverse Effects

Recommended Dosing

Pediatric Dosing

For serious infections such as infective endocarditis, ampicillin is dosed at 200–300 mg/kg/day IV divided every 4–6 hours, with a maximum of 12 g daily. 1

• For relatively penicillin-resistant streptococci and enterococci, ampicillin 200–300 mg/kg/day IV divided every 4–6 hours (up to 12 g daily) is combined with gentamicin for synergistic effect 1
• The oral formulation of ampicillin has been historically used for respiratory tract infections (otitis media, bronchitis, pneumonia) and urinary tract infections at lower doses, though specific tablet dosing is less commonly recommended in modern guidelines 2, 3

Adult Dosing

• For infective endocarditis with enterococci or relatively resistant streptococci, adults receive ampicillin at the same weight-based dosing (up to 12 g daily) combined with gentamicin 3–5 mg/kg/day 1
• For community-acquired intra-abdominal infections, ampicillin-sulbactam 200–300 mg/kg/day IV divided every 4–6 hours (up to 12 g daily of ampicillin component) is recommended 1

Administration Instructions

Route and Timing

• Intravenous administration is strongly preferred for serious infections, with doses divided every 4–6 hours to maintain therapeutic levels 1
• Oral ampicillin tablets should be taken on an empty stomach (1 hour before or 2 hours after meals) to maximize absorption, though this is based on general pharmacokinetic principles 2
• For endocarditis, treatment duration is typically 4–6 weeks depending on organism and valve involvement 1

Important Clinical Context

• Ampicillin-sulbactam is NOT recommended for community-acquired intra-abdominal infections due to high resistance rates among E. coli (>20% resistance in many communities) 1
• Amoxicillin has largely replaced oral ampicillin in clinical practice because it achieves higher blood levels for longer periods at half the dose, with equal in vitro activity 2

Contraindications

Absolute Contraindications

• History of serious hypersensitivity reaction (anaphylaxis, Stevens-Johnson syndrome) to any penicillin or beta-lactam antibiotic (based on general penicillin prescribing principles)
• Infectious mononucleosis or Epstein-Barr virus infection (ampicillin causes a characteristic maculopapular rash in 80–100% of these patients, though this is based on general medical knowledge)

Relative Contraindications

• Severe renal impairment requires dose adjustment and extended dosing intervals to prevent drug accumulation 4
• History of penicillin-associated colitis or severe gastrointestinal reactions warrants careful consideration of alternative agents

Common Adverse Effects

Gastrointestinal Effects

• Diarrhea and modification of intestinal transit are the most common adverse effects, occurring in a variable percentage of patients and usually resolving spontaneously 5
• Nausea and abdominal discomfort occur less frequently 5

Hypersensitivity Reactions

• Maculopapular rash occurs in 5–10% of patients receiving ampicillin, with higher rates (80–100%) in patients with infectious mononucleosis (based on general medical knowledge)
• Serious allergic reactions (anaphylaxis, angioedema) occur in <1% of patients but require immediate discontinuation

Hematologic Effects

• Reversible neutropenia and thrombocytopenia can occur with prolonged high-dose therapy, particularly in endocarditis treatment lasting 4–6 weeks 1

Critical Clinical Considerations

Resistance Patterns

• Shigellosis has become refractory to ampicillin due to widespread resistant strains, and ampicillin should not be used for this indication 2
• Ampicillin-resistant Haemophilus influenzae strains have emerged and may become a serious problem in meningitis treatment 2
• For gonorrhea, ampicillin remains effective but is no longer first-line due to resistance concerns 2

Therapeutic Monitoring

• For endocarditis, reassess clinical response at 48–72 hours; lack of improvement warrants blood culture repeat and consideration of alternative or additional antimicrobial therapy 1
• Renal function should be monitored during prolonged therapy, with dose adjustment for creatinine clearance <30 mL/min 4

Common Pitfalls to Avoid

• Do not use ampicillin-sulbactam empirically for community-acquired intra-abdominal infections due to E. coli resistance rates exceeding 20% in most communities 1
• Do not prescribe oral ampicillin when amoxicillin is available, as amoxicillin provides superior pharmacokinetics with twice the blood levels at half the dose 2
• Do not underdose serious infections; the full 200–300 mg/kg/day (up to 12 g daily) is required for endocarditis and other severe infections 1
• Always combine ampicillin with gentamicin for enterococcal endocarditis to achieve bactericidal synergy 1