Ampicillin
Pronunciation: am-pih-SIL-in.
- Generic name
- ampicillin
- Brand names
- Principen (US, discontinued as a branded product; generic ampicillin capsules remain available)
- Drug class
- Aminopenicillin (beta-lactam antibiotic)
- Form
- Oral capsule; also available as an injectable powder for reconstitution in hospital settings
- Route
- Oral (capsule) or intravenous/intramuscular injection, product-dependent
- Legal status
- Prescription-only
Availability and supply rules vary by country
Trade names by country: Canada
Australia: · Full directory
What ampicillin is and how it works
Ampicillin is an aminopenicillin antibiotic that kills susceptible bacteria by interfering with the formation of their cell walls. It is used only for infections proven or strongly suspected to be caused by bacteria, not for viral infections such as colds or flu. (1)
Which infection ampicillin is prescribed for, the dose, and how long you take it depend on the specific infection and your individual health. Take the full course exactly as prescribed, even if you feel better before it is finished.
Mechanism of action: how ampicillin works
Ampicillin is an aminopenicillin beta-lactam antibiotic that inhibits bacterial cell-wall synthesis, producing a bactericidal effect against susceptible organisms. It is inactive against penicillinase-producing strains. (1)
What ampicillin is used for
Genitourinary tract infections, including gonorrhea
Used to treat infections of the urinary and genital tract caused by susceptible bacteria, including gonorrhea (given as a single larger dose combined with probenecid). (1)
Respiratory tract infections
Used to treat infections of the airways and lungs caused by susceptible bacteria. (1)
Gastrointestinal tract infections
Used to treat susceptible bacterial infections of the gut. (1)
Meningitis
Used, typically as an injectable product in hospital, to treat meningitis caused by susceptible bacteria. (1)
Before taking ampicillin
Do not take / not appropriate for
- Do not take ampicillin if you have had a serious hypersensitivity (allergic) reaction to ampicillin or any other beta-lactam antibiotic (including penicillins and cephalosporins). (United States label; check local product information) (1)
- Do not take ampicillin if you have previously had cholestatic jaundice or liver problems associated with ampicillin. (United States label; check local product information) (1)
Tell your clinician about
- Tell your clinician if you have ever had an allergic reaction to any penicillin or cephalosporin antibiotic.
- Tell your clinician if you have infectious mononucleosis (glandular fever) — a high proportion of people with mononucleosis develop a rash if given ampicillin, so it is not usually given in this situation.
- Tell your clinician about all other medicines you take, especially oral contraceptives, probenecid, allopurinol, or other antibiotics.
- Tell your clinician if you are pregnant, trying to become pregnant, or breastfeeding.
How to take ampicillin
- Take ampicillin capsules at least half an hour before or two hours after meals, since food reduces how much is absorbed.
- Take at evenly spaced times as directed, and finish the entire prescribed course even if you start to feel better.
- Adult dosing for genitourinary or gastrointestinal infections is commonly 500 mg four times daily; for respiratory tract infections it is commonly 250 mg four times daily. Your prescriber's instructions and the exact product label take priority over these general figures.
- Gonorrhea is typically treated with a single larger 3.5 g dose combined with 1 g of probenecid, exactly as directed by your prescriber.
Procedures and treatment changes
Tell the treating team before surgery or any procedure that you are taking ampicillin. Do not alter your treatment without the responsible clinician. (1)
Missed dose
If you miss a dose, take it as soon as you remember, unless it is almost time for your next dose — in that case, contact your prescriber or pharmacist for guidance on the missed dose and continue your normal schedule. do not take an extra dose to make up for a missed one.
Overdose
Ampicillin overdose is not well characterised in the label reviewed for this page. If an overdose is suspected, contact a poison-control or emergency service, or go to the nearest emergency department, and take the medicine packaging with you. (1)
Side effects
Side effects and their urgency vary; report severe, persistent, or concerning symptoms to a clinician or pharmacist.
Diarrhea, nausea, vomiting, or mouth soreness (stomatitis)
What to notice: Mild stomach upset is relatively common with ampicillin.
What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (1)
Rash
What to notice: A skin rash can occur, and is especially common in people with infectious mononucleosis given ampicillin.
What to do: Tell your clinician about any new rash, since it needs to be assessed to distinguish an ordinary rash from an allergic reaction. (1)
Severe or watery diarrhea, including diarrhea that occurs weeks after finishing treatment
What to notice: Watery or bloody stools, stomach cramps, or fever after or during ampicillin treatment
What to do: Contact a clinician promptly — this can signal Clostridioides difficile-associated colitis, which can range from mild to life-threatening and sometimes needs specific treatment. (1)
Signs of liver problems
What to notice: Yellowing of the skin or eyes, dark urine, unusual tiredness, or pain in the upper right abdomen
What to do: Contact a clinician promptly for assessment of possible liver involvement. (1)
Serious warnings
Anaphylaxis and serious allergic reactions — urgent
Symptoms: Swelling of the face, lips, tongue, or throat; difficulty breathing; widespread hives; feeling faint or collapsing
Action: Seek emergency medical care immediately. Serious and occasionally fatal hypersensitivity reactions have been reported with penicillin-class antibiotics, including ampicillin. (1)
Severe skin reactions (Stevens-Johnson syndrome, toxic epidermal necrolysis, DRESS syndrome) — urgent
Symptoms: Blistering or peeling skin, widespread rash, fever, swollen glands, or facial swelling
Action: Seek urgent medical care immediately for any severe or blistering skin reaction. (1)
C. difficile-associated diarrhea/colitis — serious
Symptoms: Watery or bloody diarrhea, abdominal pain, and fever, which can begin during treatment or up to several weeks after stopping
Action: Contact a clinician promptly if this occurs; this needs clinical assessment and sometimes specific treatment. (1)
Hepatic dysfunction and cholestatic jaundice — serious
Symptoms: Yellowing of the skin or eyes, dark urine, pale stools, or unusual tiredness
Action: Contact a clinician promptly for liver function assessment if these symptoms occur. (1)
Interactions
| Medicine or test | Effect | Action |
|---|---|---|
| Probenecid | Probenecid reduces how quickly the kidneys remove ampicillin, which raises and prolongs ampicillin blood levels and can increase the risk of toxicity. | This combination is sometimes used deliberately (e.g. for gonorrhea treatment) under medical supervision — follow your prescriber's specific instructions. (1) |
| Allopurinol | Taking allopurinol together with ampicillin increases the likelihood of developing a skin rash. | Tell your clinician if you take allopurinol so they can watch for rash. (1) |
| Oral contraceptives (birth control pills) | Ampicillin may reduce the effectiveness of oral contraceptives. | Discuss additional or backup contraception with your clinician while taking ampicillin. (1) |
| Bacteriostatic antibiotics (chloramphenicol, erythromycins, sulfonamides, tetracyclines) | These can interfere with the bactericidal (bacteria-killing) action of ampicillin when given at the same time. | Give the clinician and pharmacist a complete list of medicines so combinations like this can be avoided or managed. (1) |
Pregnancy, breastfeeding, kidney/liver function, age
Infectious mononucleosis (glandular fever)
A high proportion of people with mononucleosis who take ampicillin develop a rash, so it is not usually given in this situation. (1)
Pregnancy and breastfeeding
Penicillin-class antibiotics, including ampicillin, are generally used in pregnancy only when clearly needed. Ampicillin passes into breast milk and could sensitize a nursing infant, so discuss individual risks and benefits with your clinician. (1)
Children
Ampicillin is used in children, with dosing based on body weight (commonly 100 mg/kg/day for genitourinary/gastrointestinal infections or 50 mg/kg/day for respiratory infections, divided into multiple doses) rather than adult fixed doses. (1)
Monitoring
Before starting
- Your clinician should confirm the infection is likely bacterial and review your allergy history to penicillins and cephalosporins before starting ampicillin.
During treatment
- During prolonged courses, liver function may be monitored because hepatic dysfunction and cholestatic jaundice have been reported with ampicillin.
- Report any new rash, severe diarrhea, or signs of allergic reaction promptly during treatment.
Storage
Store ampicillin capsules as directed on the product packaging, and keep out of reach of children. Follow any specific storage or reconstitution instructions provided if you are given an injectable form.
Study the drug class: Penicillins
Professional information
Use the current jurisdiction- and product-specific prescribing information.
United States
Indication: Genitourinary and gastrointestinal tract infections, adults and children over 20 kg
Dose summary: 500 mg orally four times daily.
Renal context: The reviewed label does not specify a numeric renal dose-adjustment table; ampicillin is renally eliminated, so dose or interval adjustment is generally required in renal impairment per current local guidance and product information. (1)
United States
Indication: Respiratory tract infections, adults and children over 20 kg
Dose summary: 250 mg orally four times daily.
Renal context: The reviewed label does not specify a numeric renal dose-adjustment table; ampicillin is renally eliminated, so dose or interval adjustment is generally required in renal impairment per current local guidance and product information. (1)
United States
Indication: Gonorrhea
Dose summary: A single 3.5 g oral dose given with 1 g probenecid.
Renal context: Not separately specified for this single-dose regimen in the reviewed label. (1)
Hepatic guidance
Hepatic dysfunction and cholestatic jaundice have been reported with ampicillin; monitor liver function during prolonged therapy and avoid use in patients with a prior history of ampicillin-associated cholestatic jaundice or hepatic dysfunction.
Overdose note
The reviewed label does not provide a detailed overdose management protocol; manage per current poison-control/toxicology guidance and supportive care. (1)
Frequently asked questions
What is ampicillin used for?
Ampicillin is an antibiotic used to treat bacterial infections of the genitourinary tract (including gonorrhea), respiratory tract, gastrointestinal tract, and, as an injectable hospital product, meningitis. It only works against bacterial infections, not viral ones like colds or flu. (1)
Should I take ampicillin with food?
No — take ampicillin capsules at least half an hour before or two hours after a meal, since food reduces how much of the medicine your body absorbs. (1)
What side effects can ampicillin cause?
Common effects include diarrhea, nausea, vomiting, mouth soreness, and rash. Seek urgent care for signs of a serious allergic reaction (facial or throat swelling, difficulty breathing, widespread hives) or a severe skin reaction, and contact a clinician promptly for severe or bloody diarrhea or signs of liver problems. (1)
Can ampicillin interact with other medicines?
Yes. Probenecid raises ampicillin levels, allopurinol increases the chance of rash, ampicillin may make oral contraceptives less effective, and certain other antibiotics (chloramphenicol, erythromycins, sulfonamides, tetracyclines) can interfere with how well ampicillin works if taken at the same time. Give your clinician and pharmacist a full list of your medicines. (1)
Why might my clinician avoid ampicillin if I have mononucleosis (glandular fever)?
A high proportion of people with mononucleosis develop a rash when given ampicillin, so it is generally avoided in that situation. (1)
What is the mechanism of action of ampicillin?
Ampicillin is an aminopenicillin beta-lactam antibiotic that inhibits bacterial cell-wall synthesis, producing a bactericidal effect against susceptible organisms. It is inactive against penicillinase-producing strains. (1)
What is the generic name of Ampicillin?
The generic name is ampicillin. It is sold under brand names including Principen (US, discontinued as a branded product; generic ampicillin capsules remain available). (1)
What class of drug is ampicillin?
Ampicillin is classed as: Aminopenicillin (beta-lactam antibiotic). (1)
References
- AMPICILLIN capsule — full prescribing informationU.S. Food and Drug Administration / DailyMed (NIH) · United States · accessed 2026-08-05 · source 1
- AMPICILLIN AND SULBACTAM (AMPICILLIN SODIUM AND SULBACTAM SODIUM) INJECTION, POWDER, FOR SOLUTION [MEITHEAL PHARMACEUTICALS INC.] — US prescribing information (DailyMed SPL)U.S. National Library of Medicine (DailyMed) · US · accessed 2026-08-14 · source 2
- Ampicillin and Sulbactam — FDA approval record (Drugs@FDA ANDA065406)U.S. Food and Drug Administration · US · accessed 2026-08-14 · source 3
- ampicillin — RxNorm normalized drug concept (RXCUI 203115)U.S. National Library of Medicine (RxNorm) · US · accessed 2026-08-14 · source 4
Brand names by country
Single-ingredient ampicillin names are shown separately by country. Product availability, strengths, and supply rules can change; combination products are not included here.
Priority countries
Canada
- APO AMPI