Ampicillin: What It Treats, How to Use It, and What to Watch For

Ampicillin is an older penicillin antibiotic that still helps treat many infections. Want a quick, no-nonsense run-through? Here’s what you need to know: where it works, common side effects, how people usually take it, and simple safety tips.

What ampicillin treats and how it works

Ampicillin fights bacteria by stopping them from building their cell walls. That makes it useful for infections caused by certain gram-positive and some gram-negative bacteria. Doctors commonly use it for ear, throat, sinus, bladder/urinary infections, and some gut infections. It’s also a go-to drug for Listeria infections and is available as pills, a liquid you swallow, or an injection for hospital use.

Not every infection needs ampicillin. It won’t help viral illnesses like colds or most sore throats from viruses. Lab testing (culture and sensitivity) often guides whether ampicillin is a good choice, because some bacteria are resistant to it.

Dosage, common side effects, and practical tips

Typical adult oral doses are often in the range of 250–500 mg every 6 hours, but your doctor will pick the exact dose based on the infection and your health. For kids, dosing usually depends on weight. For serious infections, hospitals use higher intravenous doses. Don’t change dose or stop early without talking to your prescriber.

Common side effects are stomach upset, diarrhea, and rash. Serious allergic reactions to penicillins can happen—look for hives, swelling, breathing trouble, or fainting—those need emergency care. Another risk is Clostridioides difficile (C. diff) infection after antibiotics, which causes severe diarrhea; see a doctor if diarrhea is heavy or bloody.

Some drugs interact with ampicillin. Probenecid can raise antibiotic levels. If you take immunosuppressants or chemotherapy drugs, mention antibiotics to your care team. People who know they’re allergic to penicillin should never take ampicillin.

Few practical tips: take doses evenly through the day, with or without food as you tolerate. Finish the full course unless your doctor tells you otherwise—stopping early can allow bacteria to grow back and become resistant. Store capsules and tablets in a cool, dry place; liquid suspensions often need refrigeration after mixing and should be used within the label time.

Pregnant or breastfeeding? Ampicillin is often used in pregnancy when needed, but always tell your obstetrician before taking any antibiotic. If you suspect a drug allergy or you get unusual symptoms, stop the drug and contact a healthcare provider right away.

If you think you need ampicillin, get checked by a clinician who can confirm the infection and pick the right dose. Avoid buying antibiotics without a prescription—wrong drug or wrong dose can do more harm than good.