Ampicillin: What It Is and How to Use It Safely

Ampicillin is a penicillin-family antibiotic used to fight many common bacterial infections. It works by stopping bacteria from building cell walls, which kills them. Doctors prescribe it for ear and throat infections, urinary tract infections, some types of pneumonia, and certain gastrointestinal and bloodstream infections. It's available as oral capsules, liquid suspension, and by injection for hospital use.

Take ampicillin exactly as your prescriber tells you. For oral forms, absorption is best on an empty stomach — take it one hour before or two hours after meals unless your doctor says otherwise. Finish the full course even if you feel better; stopping early can let bacteria come back and breed resistance. If you miss a dose, take it as soon as you remember but skip it if the next dose is near.

Watch for allergic reactions. If you get a rash, hives, swelling of the face or throat, or trouble breathing, stop the drug and seek emergency care. People with a history of penicillin allergy may react badly to ampicillin; tell your provider about any past allergic reactions. Also mention a history of mono (infectious mononucleosis): ampicillin commonly causes a rash in that condition.

Common side effects are nausea, diarrhea, and mild stomach upset. If diarrhea is severe or contains blood, tell your doctor — antibiotics can trigger C. difficile, a serious gut infection. Less common effects include yeast infections and changes in blood counts. Keep a list of all your medications handy, because ampicillin can interact with other drugs — for example, it can affect blood thinner dosing and interact with some seizure medicines.

Pregnant and breastfeeding people often can take ampicillin, but check with your provider first. Pediatric dosing depends on age and weight; caregivers should follow dosing instructions and measure liquid medicine with the provided device. For injectable ampicillin in hospitals, trained staff will handle dosing and monitoring.

Store tablets and capsules at room temperature away from moisture. Keep liquid suspensions refrigerated if the label says so and throw away leftovers after the recommended time. Never share antibiotics with others or use leftover pills for a new illness.

If you buy medications online, use a licensed pharmacy and require a prescription. Beware of sites that sell antibiotics without asking for a prescription — those products may be fake or unsafe. Ask your pharmacist about generic ampicillin options, side effects to watch for, and how the medicine fits your other treatments.

When to call your doctor: high fever, worsening symptoms, new rash, difficulty breathing, severe diarrhea, or signs of liver problems like yellowing skin. Keep clear communication with your prescriber and pharmacist so ampicillin helps you get better without unnecessary risks.

If symptoms don't improve within 48–72 hours, contact your doctor. They may order a culture or switch antibiotics. Resistance to ampicillin is common in some bacteria, so lab testing helps pick the right drug. Keep vaccination and hygiene up to reduce infections. Store records of antibiotic use and ask your provider about allergy testing if you think you reacted to penicillin.