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Aminopenicillin Antibiotic

Ampicillin

Brand names: Penbritin

Adult dose

Dose: Oral: 250–500 mg every 6 hours. IV/IM: 500 mg–1 g every 4–6 hours; higher doses for severe infections (e.g., listeria meningitis: 2 g IV every 4 hours)
Route: Oral, intravenous, or intramuscular
Frequency: Every 6 hours (oral/IM); every 4–6 hours (IV)

Clinical pearls

  • Superseded by amoxicillin for most oral indications (better oral bioavailability)
  • IV ampicillin remains important for listeria meningitis, enterococcal endocarditis, and Group B streptococcal infections
  • The ampicillin rash in EBV is maculopapular and not a true penicillin allergy — does not predict future penicillin allergy
  • EUCAST/BSAC breakpoints: check local sensitivity, as many organisms are resistant
  • For endocarditis: ampicillin + gentamicin for enterococcal species (BSAC/BCS guidelines)

Contraindications

  • Penicillin allergy (severe hypersensitivity — anaphylaxis history)
  • Infectious mononucleosis (glandular fever) — high risk of ampicillin rash (maculopapular)

Side effects

  • Rash (10% of patients; ~90% if glandular fever — maculopapular, not true allergy)
  • Diarrhoea (including antibiotic-associated Clostridioides difficile)
  • Nausea
  • Hypersensitivity (urticaria, anaphylaxis — rare but serious)
  • Seizures at very high doses in renal impairment

Interactions

  • Methotrexate — penicillins reduce methotrexate excretion; risk of toxicity
  • Warfarin — may alter gut flora and increase anticoagulant effect
  • Oral typhoid vaccine — ampicillin may reduce efficacy (avoid within 3 days of live typhoid vaccine)
  • Allopurinol — increases risk of rash with ampicillin

Monitoring

  • Clinical response within 48–72 hours
  • Renal function (dose adjust if eGFR <30)
  • Signs of Clostridioides difficile infection
  • LFTs in prolonged courses

Reference: BNF; BSAC Endocarditis Guidelines (2012 updated); NICE NG51 (Meningitis); PHE/UKHSA antibiotic guidelines; https://bnf.nice.org.uk/drugs/ampicillin/. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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