Prescription ·Strong evidence ·Reviewed May 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.
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 5 interaction records on file.
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
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.
Nutrients this medication can lower over time, and what to replace.
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.
Ampicillin can suppress or kill bacterial probiotic organisms if taken at the same time, although selected probiotics may lower antibiotic-associated diarrhea risk.
Recommendation: Separate probiotic doses from the antibiotic by at least 2 hours when feasible; avoid probiotic use in severely immunocompromised patients or patients with central lines unless clinician-directed.
Prolonged Ampicillin therapy can contribute to reduced vitamin K status in susceptible patients by altering gut flora, with higher concern in poor intake, malabsorption, liver disease, or warfarin use.
Recommendation: Do not self-treat bleeding or INR changes. Monitor for bruising or bleeding and coordinate vitamin K or anticoagulant changes with the prescriber.
Broad-spectrum antibiotics such as ampicillin can suppress vitamin K-producing gut bacteria, occasionally lowering vitamin K status. This is most clinically relevant in patients on warfarin, where prolonged antibiotic courses plus poor dietary vitamin K intake can potentiate anticoagulant effect and raise bleeding risk. Vitamin K2 supplementation may stabilize vitamin K status.
Recommendation: Routine vitamin K2 supplementation is not required for short ampicillin courses in well-nourished patients. In patients on warfarin or with poor nutritional intake, monitor for signs of vitamin K deficiency (easy bruising, bleeding) and keep vitamin K intake consistent; do not start or stop vitamin K abruptly while on warfarin. Discuss any vitamin K supplement with the prescriber managing anticoagulation.
Although chelation interactions with penicillins are far less pronounced than with tetracyclines or fluoroquinolones, divalent cations such as zinc can form complexes with some beta-lactam antibiotics and may modestly reduce absorption when taken together. The clinical impact for ampicillin is generally small.
Recommendation: To avoid any reduction in antibiotic absorption, take zinc supplements at least 2 hours before or after ampicillin. Ampicillin is also best taken on an empty stomach (1 hour before or 2 hours after food) for optimal absorption.
Calcium is a divalent cation that can theoretically chelate beta-lactam antibiotics, and large meals or dairy can reduce ampicillin absorption. Ampicillin bioavailability is improved on an empty stomach. The chelation effect with calcium is much weaker than with tetracyclines and is rarely clinically significant for ampicillin.
Recommendation: Take ampicillin on an empty stomach, at least 1 hour before or 2 hours after food and calcium-containing supplements or dairy products, to maximize absorption. Separate calcium supplements from ampicillin by about 2 hours.
Numbered references. Citations throughout the page link here.
Ampicillin plus an aminoglycoside (or ampicillin plus ceftriaxone for E. faecalis) is recommended for enterococcal infective endocarditis.
Ampicillin (often with an aminoglycoside) is the recommended agent for Listeria monocytogenes meningitis.
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.
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