Overview
Active ingredient: ampicillin (commonly supplied as ampicillin trihydrate)
Dosage strengths available: 250 mg and 500 mg capsules for oral use
Drug class: aminopenicillin, a broad-spectrum beta-lactam antibiotic
Ampicillin works by blocking bacterial cell wall synthesis, which leads to the death of susceptible bacteria. It is stable in stomach acid, which allows effective oral absorption when taken on an empty stomach. This medicine targets many gram-positive organisms and several gram-negative bacteria, including certain strains of E. coli, Proteus mirabilis, Salmonella, Shigella, Klebsiella pneumoniae, and Haemophilus influenzae. Bacteria that produce penicillinase or many beta-lactamases are typically resistant.
For details on how to take it, jump to Dosage and directions. For a comparison with related antibiotics, see Alternatives.
Indications and common uses
Ampicillin is prescribed to treat bacterial infections proven or strongly suspected to be caused by susceptible organisms. Examples include:
- Respiratory tract infections such as community-acquired pneumonia, bronchitis, and sinusitis
- Urinary tract infections
- Gastrointestinal infections including some cases of typhoid and shigellosis
- Ear, nose, and throat infections
- Selected skin and soft tissue infections
- In combination therapy for certain enterococcal infections
It does not treat viral illnesses such as the common cold or influenza. Your clinician may order cultures or local susceptibility guidance to confirm that ampicillin is appropriate.
Dosage and directions
Use exactly as prescribed by your healthcare professional. Typical oral dosing for adults varies by infection type and severity. Common practice is to divide the daily dose into 3 or 4 evenly spaced administrations.
- Take ampicillin on an empty stomach: 30 minutes before food or 2 hours after meals.
- Swallow capsules with a full glass of water.
- Space doses evenly through the day to maintain steady blood levels.
- If you have kidney impairment, your prescriber may adjust the dose interval.
Complete the full course even if you feel better, unless your prescriber tells you to stop. Stopping early can lead to relapse or resistance.
For missed doses, see Missed dose.
Warnings and precautions
Before starting therapy, tell your clinician about all allergies, conditions, and medicines. Important considerations include:
- History of allergy to penicillins or cephalosporins may predict serious reactions. Seek urgent care if you develop hives, swelling of the face or throat, trouble breathing, or severe rash.
- Severe diarrhea or watery/bloody stools can signal C. difficile colitis. Stop the drug and get medical help if this occurs.
- Prolonged or repeated use may cause overgrowth of non-susceptible organisms, including yeast infections.
- Ampicillin can cause a false positive in certain urine glucose tests using reducing substances. Enzyme-based glucose oxidase tests are generally unaffected.
- Pregnancy and lactation: ampicillin has long clinical experience in pregnancy and is commonly used when indicated. It passes into breast milk and may cause diarrhea or thrush in the infant; monitor and consult your clinician.
- People with asthma, atopy, or multiple drug allergies may have higher risk of hypersensitivity reactions.
- Liver disease or kidney disease may require monitoring or dosage adjustments.
For interacting medicines, see Drug interactions.
Who should not use ampicillin
- Known hypersensitivity to ampicillin, any penicillin, or formulation components
- Previous severe beta-lactam reaction such as anaphylaxis or severe cutaneous adverse reaction
Use caution and seek specialist input if you have a history of severe immediate reactions to cephalosporins.
Possible side effects
Most people tolerate ampicillin well. Report troublesome or severe effects to your clinician. Potential effects include:
- Gastrointestinal upset: nausea, vomiting, diarrhea, abdominal pain
- Rash or itching; hives; drug fever
- Oral thrush or vaginal yeast overgrowth
- Colitis including C. difficile associated diarrhea
- Sore tongue or mouth
- Rare: liver enzyme elevations, hematologic changes, seizures in high doses with renal impairment, severe allergic reactions
If you experience signs of a severe allergy such as swelling of lips or throat, wheezing, or widespread blistering rash, call emergency services right away.
To understand if a symptom could be due to interactions, see Drug interactions.
Drug interactions
Ampicillin can interact with other medicines and lab tests:
- Warfarin and other anticoagulants: antibiotics may increase INR in some patients. Monitor as directed.
- Oral contraceptives: decreased efficacy has been reported rarely; consider a backup method while on therapy and for 7 days after.
- Allopurinol: increased risk of rash when combined.
- Probenecid: raises ampicillin blood levels by decreasing renal excretion.
- Aminoglycosides: potential synergy against some organisms when given separately; do not mix in the same IV solution.
Always provide a full medication list to your prescriber, including OTC drugs and supplements.
Missed dose
If you forget a dose, take it as soon as you remember unless it is nearly time for the next one. If it is close to the next dose, skip the missed dose and resume your regular schedule. Do not double up to make up a missed dose.
Keeping doses evenly spaced helps maintain effectiveness. For timing guidance, revisit Dosage and directions.
Overdose
Symptoms can include severe nausea or vomiting, diarrhea, dizziness, confusion, or seizures, particularly if kidney function is reduced. In the USA, contact Poison Control at 1-800-222-1222 or use poisonhelp.org, and seek immediate medical assistance.
Storage
- Store capsules at controlled room temperature, ideally 68 to 77 F (20 to 25 C); brief excursions 59 to 86 F (15 to 30 C) are generally acceptable.
- Keep tightly closed, protected from moisture and excessive heat, and away from direct light.
- Keep out of reach of children and pets.
Alternatives and similar medications
Depending on the infection, your clinician may consider other antibiotics with overlapping or broader/narrower coverage. Choice depends on culture results, local resistance patterns, allergy history, and site of infection.
- Amoxicillin: similar spectrum with generally better oral absorption; often dosed less frequently.
- Amoxicillin-clavulanate (Augmentin): adds beta-lactamase inhibition for organisms resistant to ampicillin alone.
- Ampicillin-sulbactam (Unasyn, IV): hospital therapy adding beta-lactamase inhibitor for mixed infections.
- Penicillin V: narrower spectrum, often for streptococcal pharyngitis when susceptible.
- Cephalexin: a first-generation cephalosporin frequently used for skin and urinary infections.
- Azithromycin or doxycycline: alternative classes sometimes used for respiratory infections or in penicillin-allergic patients, depending on susceptibility.
- Cefuroxime or cefdinir: oral cephalosporins with respiratory and some urinary coverage.
Only a clinician can determine whether a given alternative is appropriate for your specific infection. See also the price comparison for a rough cost overview in the USA.
Typical retail prices in the USA (USD)
Prices vary widely by pharmacy, brand, dose, quantity, insurance coverage, and use of discount programs. The ranges below are rough cash estimates for common fills as of recent market observations:
- Ampicillin 500 mg capsules, 28 to 40 count: roughly $12 to $45
- Ampicillin 250 mg capsules, 28 to 40 count: roughly $10 to $35
- Amoxicillin 500 mg capsules, 20 to 40 count: roughly $5 to $20
- Amoxicillin-clavulanate 875/125 mg, 14 to 20 count: roughly $12 to $60
- Cephalexin 500 mg, 28 to 40 count: roughly $8 to $30
- Azithromycin 250 mg, 6-tablet Z-Pak: roughly $8 to $25
Using pharmacy discount cards or price-comparison tools can significantly reduce out-of-pocket costs. Hospital IV formulations (for example, ampicillin-sulbactam) are billed differently and are not reflected in these retail estimates.
Legal status and purchasing rules in the USA
- Prescription only: In the United States, ampicillin is an FDA-approved prescription medication under the Federal Food, Drug, and Cosmetic Act. It cannot be legally sold over the counter for human use.
- Valid prescription required: Dispensing must follow state Board of Pharmacy regulations. Telehealth prescriptions are allowed when compliant with federal and state laws.
- Importation: Personal importation of prescription drugs not approved for US distribution is generally prohibited. Exceptions are narrow and discretionary.
- Use as directed: Using antibiotics without a prescription, using non-human formulations (for example, fish or veterinary antibiotics) for human treatment, or sharing prescription antibiotics may violate federal or state law and is unsafe.
- Stewardship: Clinicians should prescribe antibiotics only when bacterial infection is likely or proven to reduce resistance and protect public health.
For safe and lawful access, consult a licensed US clinician and fill at a state-licensed pharmacy. For medical guidance on taking the medicine, see Dosage and directions and Warnings and precautions.
Disclaimer
The information on this page is general and may not include every precaution, interaction, or side effect. It is not a substitute for professional medical advice, diagnosis, or treatment. Always follow the specific instructions provided by your healthcare professional and the medicine label. The publisher is not responsible for any consequences resulting from reliance on this information or self-treatment.
