Ampicillin is an antibiotic used to treat bacterial infections such as urinary tract, respiratory, and gastrointestinal infections, as well as meningitis. It works by stopping the growth and spread of bacteria in the body.

Ampicillin belongs to the penicillin group of antibiotics and is available as 250 mg or 500 mg capsules, oral suspensions of 125 mg/5 mL or 250 mg/5 mL, and injectable forms of 250 mg to 2 g vials. It is marketed under brand names such as Amcill, Principen, or Omnipen, as well as in combination with sulbactam under the brand name Unasyn.

This medication should always be used under medical supervision and for the entire duration prescribed. Using ampicillin improperly or stopping treatment early can increase the risk of bacterial resistance, making infections harder to treat.

young woman taking a pill with a glass of water

Indications for use

Ampicillin is FDA-approved for the treatment of several types of bacterial infections, including:

Respiratory tract infections caused by Haemophilus influenzae (non–beta-lactamase-producing strains), Streptococcus pneumoniae, and pneumococci;
Urinary tract infections caused by Escherichia coli (E. coli), Proteus mirabilis, and enterococci;
Gastrointestinal infections caused by Salmonella and Shigella species;
Bacterial meningitis caused by Neisseria meningitidis or Listeria monocytogenes;
Endocarditis and septicemia caused by susceptible bacteria.

In cases of suspected bacterial meningitis, intravenous treatment may be started while awaiting laboratory confirmation of infection.

Dosing instructions

The dose of ampicillin should always be prescribed by a healthcare provider, considering factors such as age, body weight, and infection severity. Dosages also vary according to the medication form.

1. Ampicillin capsules (250 mg or 500 mg)

Adults and adolescents: 250 to 500 mg every 6 hours, taken on an empty stomach (30 to 60 minutes before meals or 2 hours after);
Duration of treatment: Usually 7 to 10 days, but this may vary depending on the type and severity of the infection.

2. Ampicillin oral suspension (125 mg/5 mL or 250 mg/5 mL)

The oral suspension is typically used for children, with the dose based on body weight and infection type.

Type of infection	Pediatric dosage
Respiratory	25–50 mg/kg/day in divided doses every 6 hours
Gastrointestinal	50–100 mg/kg/day in divided doses every 6 hours
Urinary tract	50–100 mg/kg/day in divided doses every 6 hours
Bacterial meningitis	100–200 mg/kg/day in divided doses every 4–6 hours

To prepare the suspension, add the amount of water indicated on the package, shake well before each use, and store it at room temperature away from direct sunlight. The reconstituted suspension is stable for 7 days and should be discarded afterward.

3. Injectable ampicillin

Ampicillin for injection can be administered intramuscularly (IM) or intravenously (IV). It is typically used in hospitals for more serious infections.

Type of infection	Adults	Children
Respiratory	250–500 mg every 6 hours	25–50 mg/kg/day every 6 hours
Gastrointestinal	500 mg every 6 hours	50–100 mg/kg/day every 6 hours
Urinary tract	500 mg every 6 hours	50–100 mg/kg/day every 6 hours
Bacterial meningitis	2–3 g every 4 hours (IV)	150–200 mg/kg/day divided every 3–4 hours
Endocarditis or sepsis	1–2 g every 4–6 hours (IV/IM)	50–200 mg/kg/day divided every 4–6 hours

Dosages and treatment duration may vary based on the infection and the patient’s condition. All injectable doses should be administered under medical supervision.

4. Ampicillin with sulbactam (Unasyn)

Ampicillin combined with sulbactam is available only for injection and is used for infections caused by beta-lactamase–producing bacteria.

Adults: 1.5 to 3 g every 6 hours (given as 1 g ampicillin + 0.5 g sulbactam per dose);
Newborns and children: 150 mg/kg/day (100 mg ampicillin + 50 mg sulbactam per kg), divided every 6 hours.

Side effects

The most common side effects of ampicillin include diarrhea, nausea or vomiting, and skin rash.

Less frequent side effects may include stomach discomfort or upper abdominal pain, hives or itching, and allergic reactions such as swelling or difficulty breathing

Seek medical help immediately if you experience symptoms of a severe allergic reaction.

Contradictions for use

Ampicillin should not be used by people who have:

An allergy to ampicillin or other penicillin antibiotics;
An allergy to any beta-lactam antibiotics such as cephalosporins;
Infections caused by beta-lactamase–producing organisms resistant to ampicillin;
Viral infections such as mononucleosis, leukemia, or other hematologic conditions;
Concurrent use of certain medications such as allopurinol, probenecid, or oral contraceptives without medical advice.

Pregnant individuals may use ampicillin when prescribed by a healthcare provider, as studies have not shown evidence of harm to the fetus.

Ampicillin is also considered safe to use while breastfeeding because only small amounts pass into breast milk. However, infants should be monitored for mild side effects such as diarrhea, rash, or thrush.

Ampicillin: Indications for Use, Dosing Instructions & Side Effects