Recommended Dosage of Oral Ampicillin for Adults
The standard recommended dosage of oral ampicillin for adults is 250-500 mg every 6 hours, with a total daily dose of 1-2 grams. 1
Dosage Considerations by Indication
Ampicillin dosing varies based on the type and severity of infection:
Standard Infections
- Mild to moderate infections: 250-500 mg every 6 hours
- Severe infections: 500 mg every 6 hours
Specific Conditions
- Streptococcal pharyngitis: 500 mg 2-3 times daily for 10 days 2
- Enterococcal infections: When susceptible to penicillin, ampicillin 12 g IV daily continuously or in 6 divided doses (for parenteral therapy) 2
- Endocarditis prophylaxis: 2.0 g as a single dose administered intramuscularly or intravenously within 30 minutes before a procedure 1
Pharmacokinetic Considerations
Oral ampicillin has important pharmacokinetic properties that affect dosing:
- Absorption limitations: Unlike amoxicillin and ampicillin esters, oral ampicillin shows a non-linear relationship between dose and serum concentration 3
- Bioavailability: Increasing the oral dose of ampicillin only partially compensates for its poor absorption 3
- Peak serum levels: The increase in peak serum levels with higher doses is less than proportional 3
Dosing Frequency
The standard dosing frequency for oral ampicillin is every 6 hours due to its relatively short half-life. This differs from amoxicillin/clavulanate which can be dosed every 8-12 hours 4.
Important Clinical Considerations
- Duration of therapy: Typically 7-10 days for most infections; longer durations (4-6 weeks) may be required for serious infections like endocarditis 2
- Bactericidal activity: Ampicillin is bactericidal rather than bacteriostatic, making it appropriate for serious infections 2
- Resistance patterns: Consider local resistance patterns when prescribing ampicillin, particularly for empiric therapy
Potential Pitfalls and Caveats
- Compliance challenges: The four-times-daily dosing regimen may reduce patient adherence compared to medications with less frequent dosing
- Absorption variability: Food can decrease absorption; ampicillin should ideally be taken on an empty stomach (1 hour before or 2 hours after meals)
- Cross-reactivity: Patients with penicillin allergies should not receive ampicillin due to cross-reactivity
- Resistance concerns: Beta-lactamase producing organisms are resistant to ampicillin; consider ampicillin-sulbactam in settings where beta-lactamase producers are common
Remember that ampicillin has largely been replaced by amoxicillin for many oral indications due to amoxicillin's superior oral absorption, less frequent dosing requirements, and improved gastrointestinal tolerance.