Ampicillin: What You Need to Know

Ampicillin is an older penicillin-type antibiotic still used for many infections. It can treat ear infections, certain respiratory and urinary tract infections, and some abdominal or skin infections. This page gives practical, plain-language facts so you know when ampicillin might be used, what to watch for, and how to take it safely.

What ampicillin treats and how it works

Ampicillin works by stopping bacteria from building their cell walls, which kills the bacteria. Doctors may prescribe it for infections caused by bacteria that are known to be sensitive to ampicillin. Labs often test the bacteria first (culture and sensitivity) to make sure ampicillin will work. If the bacteria are resistant, your provider will switch to a different antibiotic.

Common uses include middle ear infections (otitis media), sinus infections, bladder infections, and some cases of bronchitis or pneumonia. It’s also used in certain surgical settings or for specific types of bacterial meningitis when recommended by a specialist.

Dosage, taking it, and missed doses

Dosing varies by age, weight, and the infection. For adults taking oral ampicillin, typical doses are 250–500 mg every 6 hours, but your doctor will choose the right dose for you. Children’s doses are based on weight. Some infections require IV ampicillin given in a hospital. Always follow the prescription you were given.

If you miss a dose, take it as soon as you remember unless it’s close to the next dose. Don’t double up to catch up. Finish the full course even if you feel better—stopping early can let bacteria come back and breed resistance.

Store the capsules or tablets at room temperature, away from moisture and heat. If you have liquid ampicillin, follow the label—some need refrigeration or use within a set time after mixing.

Side effects, interactions, and safety tips

Common side effects are nausea, diarrhea, and mild stomach upset. If you get a rash, sudden swelling, trouble breathing, or severe diarrhea (especially with blood), call your provider or seek emergency care—these can be signs of allergy or a serious reaction.

Ampicillin is a penicillin, so anyone with a known penicillin allergy should avoid it. Tell your doctor about allergies to penicillins or cephalosporins. Also mention other meds you take—some drugs change how antibiotics work or increase side effects.

Pregnant or breastfeeding? Ampicillin is often used in pregnancy, but check with your provider first. For children, dosing is specific to weight—never guess.

Antibiotic resistance is real. Only use ampicillin when prescribed, don’t share it, and don’t keep leftover pills for later. If symptoms don’t improve in 48–72 hours or get worse, contact your provider—your infection may need a different treatment or further testing.

Questions about your prescription, side effects, or how long to take it? Ask your pharmacist or doctor. They can give advice tailored to your health and the infection you’re treating.