How the dose is calculated
Ampicillin dose is calculated in mg/kg/day, divided every 6 hours. Standard oral dosing covers most mild-moderate infections; the higher range reflects severe/parenteral-range dosing used in hospital settings for more serious infections.
Typical dosing by indication
| Indication | Dose | Frequency | Max |
|---|---|---|---|
| Standard oral | 50 mg/kg/day | every 6h (4×/day) | 2000 mg/day |
| Severe/parenteral-range | 100 mg/kg/day | every 6h (4×/day) | 8000 mg/day |
Clinical use
- Ampicillin covers many Gram-positive organisms and some Gram-negatives (E. coli, Listeria, Enterococcus) — it remains first-line for Listeria monocytogenes infection and is a component of empirical neonatal sepsis regimens.
- Oral bioavailability is lower and more food-affected than amoxicillin, which is why amoxicillin has largely replaced oral ampicillin for outpatient use.
- The higher parenteral-range dosing is used in hospital settings for more serious infections (e.g. meningitis-adjacent dosing, endocarditis) — always confirm the specific higher-tier dose against local IV antibiotic guidance rather than this general range.
- Skin rash is common with ampicillin in patients with concurrent viral infections (notably Epstein-Barr virus) and does not necessarily indicate true penicillin allergy.
Frequently asked questions
What is ampicillin used for?
Ampicillin is a broad-spectrum penicillin used for infections including some urinary tract infections, Listeria monocytogenes infection, and as part of empirical regimens for neonatal sepsis and meningitis.
Why is amoxicillin more commonly used than ampicillin orally?
Amoxicillin has better oral bioavailability and less food interference than ampicillin, so it has largely replaced oral ampicillin for outpatient use — ampicillin remains important in its IV/IM form for specific hospital indications.
Does a rash on ampicillin always mean a penicillin allergy?
Not necessarily — ampicillin rash is well-recognised in patients with a concurrent viral infection, particularly infectious mononucleosis (Epstein-Barr virus), and does not reliably predict future true penicillin allergy. Clinical correlation is needed.
Is ampicillin used for Listeria infection?
Yes — ampicillin remains a first-line agent for Listeria monocytogenes infection, which is one reason it is still included in some empirical regimens for neonates and immunocompromised patients.
References
- American Academy of Pediatrics Red Book — Ampicillin.
- British National Formulary for Children (BNFc) — Ampicillin.
- Puopolo KM, et al. Management of Neonates at Risk for Early-Onset Sepsis (AAP). Pediatrics. 2018.