Ampicillin: Uses, Dosage & Safety Information

Ampicillin is a prescription aminopenicillin supplied as capsules and as powder for oral suspension. Adults and children over 20 kg usually take 500 mg four times daily in equally spaced doses, on an empty stomach, and gonorrhoea has its own single-dose regimen of 3.5 g.¹

It is the penicillin most often given in pregnancy, which shapes both its use and its adverse-event record: exposure during pregnancy accounts for more reports in the FDA database than any symptom except treatment failure.³

Supply is generic. Ampicillin capsules and oral suspension are marketed by Sandoz and others.²

On an empty stomach. Food reduces absorption of oral ampicillin, so doses are taken one hour before or two hours after eating.¹

Equally spaced doses. Four times daily means roughly every six hours, including overnight where the prescriber has said so.

Finish the course. The label opens with the stewardship instruction for a reason: stopping early selects for resistance.

Tell the prescriber about any penicillin reaction before the first dose. Anaphylaxis has occurred on oral penicillins, not only injected ones.¹

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Ampicillin is indicated for infections proven or strongly suspected to be caused by susceptible bacteria, and the label opens with a stewardship instruction rather than an indication list: culture and susceptibility information should be considered when available.¹

• Genitourinary tract infections, including gonorrhoea
• Gastrointestinal tract infections
• Respiratory tract infections
• Meningitis and bacterial infections caused by susceptible Gram-positive and Gram-negative organisms

The clinical reason to choose ampicillin over amoxicillin is narrow and specific: better activity against Listeria and Enterococcus, which is why it appears in meningitis and endocarditis regimens where amoxicillin does not.

Bacteria producing beta-lactamase destroy ampicillin. Where that is likely, a combination with a beta-lactamase inhibitor is used instead — see Interactions for what that changes.

Ampicillin is prescription-only and supplied as a generic capsule or oral suspension.²

The concern worth naming is a documentation one. Ampicillin and the closely related amoxicillin produce a rash in most patients who take them during infectious mononucleosis, and that rash is frequently recorded as a penicillin allergy. A label of “penicillin allergic” carried for decades narrows treatment choices well beyond this drug, and a large share of such labels do not survive formal testing.

Anaphylaxis

Serious and occasionally fatal hypersensitivity reactions have occurred on penicillin therapy. They are more frequent after injection but have occurred on oral penicillins, and are more likely in people sensitive to multiple allergens. A history of penicillin hypersensitivity is the single most important thing to establish before the first dose.

The mononucleosis rash

A high proportion of patients with infectious mononucleosis who receive an aminopenicillin develop a rash. It is not reliably an allergy, and mislabelling it as one can close off a whole antibiotic class for life.

Superinfection

Prolonged use may permit overgrowth of non-susceptible organisms, including fungi. Persistent symptoms during treatment warrant reassessment rather than continuation.

Antibacterial-associated colitis

C. difficile-associated diarrhoea has been reported with nearly all antibacterials and can appear weeks after the course ends.

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Evidence scope

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What has been reported to the FDA

As of the FAERS data cut of 30 July 2026, the FDA Adverse Event Reporting System held 12,442 reports naming ampicillin, of which 11,075 were classed as serious — 89% of the total.³

Who was this medicine studied in?

Ampicillin predates modern trial-reporting requirements; the label sets dosing for adults and for children by weight rather than presenting enrolment demographics.¹ No Drug Trials Snapshot exists, and supply is generic.²

Who this is and is not appropriate for

It is not appropriate for anyone with a history of serious hypersensitivity to any penicillin or cephalosporin.¹ It is also the wrong choice where beta-lactamase-producing organisms are likely, since they destroy it.

It remains the right choice in specific places: Listeria and Enterococcus infections, where its activity exceeds that of amoxicillin, and obstetric prophylaxis where the organism is known.

Where to go next

• Report a side effect to FDA – MedWatch
• Read the current labelling – DailyMed
• Pregnancy exposure questions – MotherToBaby

References — verified Sep 2, 2026

1. Sandoz Inc. Ampicillin capsules and ampicillin for oral suspension — prescribing information. DailyMed SPL set ID 0a66c5c3-63bf-46e3-835e-26b555ab6954, version 18, effective July 17, 2025. dailymed.nlm.nih.gov. Accessed September 2, 2026.
2. US Food and Drug Administration. Drugs@FDA and openFDA labelling records for oral ampicillin. Accessed September 2, 2026.
3. US Food and Drug Administration. FDA Adverse Event Reporting System, queried through openFDA for medicinalproduct AMPICILLIN. Data cut 2026-07-30 · Accessed 2026-09-02.

Three sources are listed because three are cited, covering FDA-approved labelling, FDA regulatory records and FDA post-marketing data.

This page is medical information, not medical advice. Ampicillin is a prescription penicillin whose labelling warns of fatal hypersensitivity reactions. Nothing here replaces assessment by the clinician treating you.