How the dose is calculated
Ampicillin is dosed at 100 mg/kg/day (standard) or 200 mg/kg/day (severe infection — double the standard dose), given orally (PO) or intravenously (IV), divided into either 2 or 4 doses a day — choose the split that matches the prescribed frequency. The maximum daily dose depends on the route: 4 g/day by mouth, or up to 12 g/day IV.
Typical dosing by indication
| Indication | Dose | Frequency | Max (PO) | Max (IV) |
|---|---|---|---|---|
| Standard | 100 mg/kg/day | divided 2 or 4×/day | 4 g/day | 12 g/day |
| Severe infection (double dose) | 200 mg/kg/day | divided 2 or 4×/day | 4 g/day | 12 g/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 severe-infection (double) dose is used in hospital settings for more serious infections (e.g. meningitis-adjacent dosing, endocarditis) — always confirm the specific dose and interval against local IV antibiotic guidance.
- Skin rash is common with ampicillin in patients with concurrent viral infections (notably Epstein-Barr virus) and does not necessarily indicate true penicillin allergy.
- Ampicillin is also combined with the beta-lactamase inhibitor sulbactam (brand name Unasyn) to extend its spectrum to beta-lactamase-producing organisms — a different, fixed-ratio combination product from plain ampicillin.
Frequently asked questions
What is ampicillin?
Ampicillin is a broad-spectrum aminopenicillin antibiotic — a penicillin-class drug active against many Gram-positive bacteria and some Gram-negative organisms, given either orally or by IV/IM injection.
What is ampicillin used for?
Ampicillin is used for infections including some urinary tract infections, Listeria monocytogenes infection, and as part of empirical regimens for neonatal sepsis and meningitis — dosing (e.g. a 500 mg dose) depends on the indication, weight, and route.
Ampicillin 500 mg — how many days should it be taken?
Course length depends on the infection being treated, not a fixed rule for every prescription — typical courses run 7–10 days for most bacterial infections, but always follow the specific duration prescribed for the diagnosis rather than stopping early once symptoms improve.
Does ampicillin treat UTI, and for how many days?
Yes, ampicillin can treat susceptible urinary tract infections, typically for around 7 days — though local resistance patterns mean it isn't always first-line for UTI, and susceptibility testing or local guidance should confirm it's an appropriate choice.
What are the side effects of ampicillin?
The most common are gastrointestinal (diarrhoea, nausea) and skin rash. Rash is notably common in patients with a concurrent viral infection (see below) and doesn't always mean true penicillin allergy. Less common but more serious effects include hypersensitivity reactions and, rarely, C. difficile-associated diarrhoea with any prolonged antibiotic course.
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.
What is the brand name for ampicillin-sulbactam?
Unasyn is the most widely recognised brand name for ampicillin-sulbactam — a fixed-ratio combination that adds the beta-lactamase inhibitor sulbactam to extend ampicillin's spectrum, distinct from the plain ampicillin dosed on this page.
References
- World Health Organization. Model Formulary — Ampicillin.