Ampicillin Dosing Guidelines for Adults and Children
For bacterial infections in adults and children, ampicillin should be dosed at 500 mg four times daily for adults and 100 mg/kg/day divided into four equal doses for children weighing 20 kg or less. 1
Adult Dosing
- For genitourinary or gastrointestinal tract infections: 500 mg four times daily in equally spaced doses; severe or chronic infections may require larger doses 1
- For respiratory tract infections: 250 mg four times daily in equally spaced doses 1
- For gonorrhea: Single oral dose of 3.5 grams administered simultaneously with 1 gram of probenecid 1
Pediatric Dosing
For children weighing 20 kg or less:
For infective endocarditis in children:
- When used with gentamicin for relatively resistant streptococci: 200-300 mg/kg/day IV divided every 4-6 hours (up to 12 g daily) 2
Special Considerations
- Ampicillin should be administered at least 30 minutes before or 2 hours after meals for maximal absorption 1
- Therapy should continue for a minimum of 48-72 hours after the patient becomes asymptomatic 1
- For infections caused by hemolytic strains of streptococci, a minimum of 10 days' treatment is recommended to prevent rheumatic fever or glomerulonephritis 1
- For chronic urinary or gastrointestinal infections, frequent bacteriologic and clinical assessment is necessary during therapy and may be needed for several months afterward 1
- Stubborn infections may require treatment for several weeks 1
Indication-Specific Dosing
- For impetigo in pediatric patients: Amoxicillin (related to ampicillin) can be dosed at 50-75 mg/kg/day divided into 2 doses 3
- For bacterial meningitis: Lower doses of ampicillin (150 mg/kg/day) have been shown to be as effective as higher doses (400 mg/kg/day) 4
- For chronic Salmonella bacteriuria: Low-dose ampicillin at 250 mg twice daily for 4 weeks has been effective 5
Pharmacokinetic Considerations
- The pharmacokinetics of ampicillin in pediatric patients are comparable to those in adults, with a mean half-life of 0.77 hours 6
- Sputum levels of ampicillin are approximately 30-40 times lower than corresponding serum levels, which is important to consider when treating respiratory infections 7
Common Pitfalls and Caveats
- Doses smaller than those indicated should not be used due to risk of treatment failure and development of resistance 1
- Ampicillin should not be used as monotherapy for suspected MRSA infections 2
- For severe infections, parenteral therapy may be preferred initially before transitioning to oral therapy 8
- When treating infective endocarditis, combination therapy with gentamicin may be necessary for certain pathogens 2
Remember that ampicillin dosing may need to be adjusted based on the severity of infection, site of infection, and patient-specific factors such as renal function.