Ampicillin

Prescription ·Exploratory record · not publication-reviewed ·Updated July 13, 2026

Ampicillin is a broad-spectrum, semisynthetic aminopenicillin beta-lactam antibiotic used to treat susceptible gram-positive and certain gram-negative bacterial infections. It is a first-line agent for Listeria monocytogenes and enterococcal infections and is commonly used for respiratory tract, urinary tract, and gastrointestinal infections, as well as bacterial meningitis. Because it is degraded by bacterial beta-lactamases, it is often combined with a beta-lactamase inhibitor such as sulbactam to broaden coverage.

Read the sources → Compare profiles

Publication status

Exploratory record, not a reviewed medical publication.

See the editorial and medical-review policies for the publication gate.

This generated database record has not completed a claim-level review by a qualified human medical reviewer. Bibliography links and evidence labels are not approval. The page remains available for reference and is excluded from search indexing.

In short

What the research says.

A quick, data-only summary. The full evidence, dosing, and sources are below.

Ampicillin is a broad-spectrum, semisynthetic aminopenicillin beta-lactam antibiotic used to treat susceptible gram-positive and certain gram-negative bacterial infections. NutriStack rates the supporting evidence as strong. Common adult dosing is Oral: 250 to 500 mg every 6 hours. Intravenous/intramuscular: 1 to 2 g every 4 to 6 hours for serious infections; up to 2 g every 4 hours (12 g/day) for bacterial meningitis. Doses adjusted for renal impairment., though a prescriber sets the actual dose. With long-term use it can deplete vitamin k. This profile indexes 3 sources.

What it's good for

Respiratory tract infections caused by susceptible organisms³
Urinary tract infections (e.g., susceptible E. coli, Proteus, Enterococcus)³
Enterococcal infections including endocarditis (often combined with an aminoglycoside)^1,2
Listeria monocytogenes infections and meningitis^2,3
Group B streptococcal infection and intrapartum prophylaxis^2,3

What to watch for

Diarrhea
Nausea and vomiting
Maculopapular skin rash (notably high incidence in patients with infectious mononucleosis)
Known hypersensitivity to ampicillin, penicillins, or other beta-lactam antibiotics^1,2
History of severe allergic reaction (e.g., anaphylaxis, Stevens-Johnson syndrome) to any penicillin

The bottom line

Evidence rating strong. Most-documented uses: respiratory tract infections caused by susceptible organisms, urinary tract infections (e.g., susceptible e. coli, proteus, enterococcus), enterococcal infections including endocarditis (often combined with an aminoglycoside). 3 sources indexed (2004–2019), with 0 interaction records on file.

The science

How it works, mechanistically.

Core mechanism

Ampicillin is a bactericidal beta-lactam that inhibits bacterial cell wall synthesis. It binds to and inactivates penicillin-binding proteins (PBPs), the transpeptidase enzymes that catalyze the cross-linking of peptidoglycan strands in the cell wall. Disruption of cross-linking weakens the cell wall and, combined with the activity of bacterial autolysins, leads to osmotic lysis and cell death of actively dividing bacteria. The unprotonated amino group on the side chain improves penetration through the outer membrane porins of gram-negative organisms, extending its spectrum beyond that of penicillin G. Ampicillin is inactivated by beta-lactamase enzymes produced by many resistant organisms.^3,2

Class

Aminopenicillin antibiotic

Absorption

Best on an empty stomach

Dosing

Dosing & protocol.

Common range

Oral: 250 to 500 mg every 6 hours. Intravenous/intramuscular: 1 to 2 g every 4 to 6 hours for serious infections; up to 2 g every 4 hours (12 g/day) for bacterial meningitis. Doses adjusted for renal impairment.

Recommended form

Oral capsules or suspension for mild to moderate infections; intravenous or intramuscular injection for serious or systemic infections

Oral ampicillin should be taken on an empty stomach, at least 30 minutes before or 2 hours after meals, because food reduces and delays its absorption. Oral bioavailability is incomplete (roughly 40 to 50 percent) and decreases with higher doses.

Depletions

What it depletes.

Nutrients this medication can lower over time, and what to replace.

Vitamin K

Mild

Broad-spectrum aminopenicillins suppress the colonic bacteria (notably Bacteroides and other anaerobes) that synthesize menaquinones (vitamin K2). Reduced gut bacterial vitamin K production can lower the body's vitamin K pool, which is most relevant during prolonged therapy or when dietary vitamin K intake is poor. The effect is generally minor in well-nourished patients but can become clinically significant in those who are malnourished, critically ill, or concurrently taking vitamin K antagonist anticoagulants.

Replace Vitamin K1 (phylloquinone)Monitor Prothrombin time / INR (most useful in patients on warfarin or with poor nutritional status)Onset Days to a few weeks of therapy; risk rises with courses longer than about 7 to 10 days, especially with concurrent low dietary vitamin K intake

Full Ampicillin depletion and interaction guide →

Safety

Full safety detail.

Side effects

Diarrhea
Nausea and vomiting
Maculopapular skin rash (notably high incidence in patients with infectious mononucleosis)
Urticaria and hypersensitivity reactions
Anaphylaxis (rare but potentially fatal)
Clostridioides difficile-associated diarrhea and pseudomembranous colitis
Oral or vaginal candidiasis (superinfection)
Elevated liver enzymes

Contraindications

Known hypersensitivity to ampicillin, penicillins, or other beta-lactam antibiotics^1,2
History of severe allergic reaction (e.g., anaphylaxis, Stevens-Johnson syndrome) to any penicillin
Caution and frequent avoidance in patients with infectious mononucleosis due to high rash incidence
Cross-reactivity risk in patients with cephalosporin allergy

Sources

Sources, by evidence tier.

Numbered references. Citations throughout the page link here.

Reviews & position papers

1Infective Endocarditis in Adults: Diagnosis, Antimicrobial Therapy, and Management of Complications: A Scientific Statement for Healthcare Professionals From the American Heart AssociationBaddour LM, Wilson WR, Bayer AS, et al. · Circulation · 2015
Ampicillin plus an aminoglycoside (or ampicillin plus ceftriaxone for E. faecalis) is recommended for enterococcal infective endocarditis.
2Practice Guidelines for the Management of Bacterial MeningitisTunkel AR, Hartman BJ, Kaplan SL, et al. · Clinical Infectious Diseases · 2004
Ampicillin (often with an aminoglycoside) is the recommended agent for Listeria monocytogenes meningitis.

Reference material

3Ampicillin Capsules and Oral Suspension Prescribing InformationU.S. Food and Drug Administration · FDA Prescribing Information · 2019
Ampicillin is indicated for susceptible respiratory, urinary, and gastrointestinal infections, with typical oral dosing of 250 to 500 mg every 6 hours taken on an empty stomach.

FAQ

Common questions.

Quick answers about Ampicillin, drawn from the data on this page.

What is Ampicillin?

What is Ampicillin used for?

Commonly cited benefits of Ampicillin include: Respiratory tract infections caused by susceptible organisms; Urinary tract infections (e.g., susceptible E. coli, Proteus, Enterococcus); Enterococcal infections including endocarditis (often combined with an aminoglycoside); Listeria monocytogenes infections and meningitis; Group B streptococcal infection and intrapartum prophylaxis. NutriStack rates the overall evidence as strong.

What is a typical dose of Ampicillin?

A typical adult dose of Ampicillin is Oral: 250 to 500 mg every 6 hours. Intravenous/intramuscular: 1 to 2 g every 4 to 6 hours for serious infections; up to 2 g every 4 hours (12 g/day) for bacterial meningitis. Doses adjusted for renal impairment.. The commonly recommended form is Oral capsules or suspension for mild to moderate infections; intravenous or intramuscular injection for serious or systemic infections. These are general ranges, not personalized advice; confirm the right dose with a clinician.

What are the side effects of Ampicillin?

Reported side effects include diarrhea, nausea and vomiting, maculopapular skin rash (notably high incidence in patients with infectious mononucleosis), urticaria and hypersensitivity reactions, anaphylaxis (rare but potentially fatal), clostridioides difficile-associated diarrhea and pseudomembranous colitis. This is educational information; check with a healthcare professional before use.

How strong is the evidence for Ampicillin?

NutriStack rates this as Strong, meaning multiple high-quality meta-analyses or systematic reviews converge on the same finding. Full tier criteria are at https://nutristackapp.com/methodology/evidence-tiers.

Use this with your stack

Ampicillin in NutriStack.

Add it to your stack, see how it interacts with everything else you take, and get a Stack Score that updates the moment it does.

Download on the App Store → Read the methodology