Ampicillin

Key facts

• Ampicillin is a penicillin-type antibiotic that kills bacteria by weakening their cell wall, used for chest, ear, throat, urinary and some skin and gut infections.
• It's taken by mouth or given by injection, usually several times a day. Finish the full course even if you feel better early, to stop the infection returning and resistance developing.
• Ampicillin must never be given to anyone with a confirmed penicillin allergy, since it can trigger a severe, life-threatening allergic reaction.
• Seek urgent care for facial swelling, difficulty breathing, or a widespread rash.

What Ampicillin treats

Ampicillin treats bacterial infections including chest infections, ear and throat infections, urinary tract infections, some gut infections such as typhoid and Shigella, and meningitis or bloodstream infections caused by susceptible bacteria in hospital settings. It only works against bacteria and does not treat colds, flu or other viral illnesses.

How Ampicillin works

Bacteria build a rigid cell wall to survive. Ampicillin blocks the enzymes bacteria use to build and repair this wall, so the wall weakens as the bacteria try to grow and divide. Weakened bacteria burst and die, clearing the infection while your immune system deals with the remainder.

Before you take it

• Do not take ampicillin if you've had a serious allergic reaction to any penicillin or cephalosporin antibiotic.
• Tell your prescriber about kidney disease, since the dose may need adjusting, and about glandular fever (infectious mononucleosis), which almost always causes a widespread rash with ampicillin.
• Ampicillin can reduce the effectiveness of hormonal contraceptive pills; use an additional barrier method during treatment and for a few days after.
• Tell your prescriber about other medicines, particularly allopurinol, which raises the chance of a rash when combined with ampicillin.

Side effects

Common effects include nausea, diarrhoea, mild rash and headache.

Stop and seek urgent medical care for:

• Facial or throat swelling, difficulty breathing, or a rapidly spreading rash.
• Severe or bloody diarrhoea, which can signal a serious gut infection linked to antibiotic use.
• Unusual bruising or bleeding.

Safety essentials

• Confirmed penicillin allergy is an absolute contraindication. Always tell every prescriber and pharmacist about any past penicillin reaction before taking ampicillin or any related antibiotic.
• Finish the entire prescribed course, even once symptoms improve, to fully clear the infection and reduce the risk of resistant bacteria developing.
• Seek care promptly for severe diarrhoea during or after treatment, since antibiotics can trigger a serious Clostridioides difficile gut infection.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.