Drug class · Respiratory

Penicillins

Also known as: beta-lactam antibiotics, amoxicillin family

The oldest family of antibiotics and still the first choice for many chest, ear, throat, skin and dental infections: amoxicillin, co-amoxiclav, flucloxacillin and benzylpenicillin.

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Biological target
Penicillin-binding proteins
Also called
beta-lactam antibiotics, amoxicillin family

Mechanism of action

Penicillins bind the enzymes bacteria use to cross-link the peptidoglycan of their cell wall; growing bacteria with a weakened wall burst. Human cells have no cell wall, which makes the class remarkably selective. Bacteria resist by making beta-lactamase enzymes that destroy the drug; clavulanic acid in co-amoxiclav blocks the enzyme, and flucloxacillin is built to withstand the staphylococcal version. They are useless against viruses.

Members of the class

amoxicillinco-amoxiclavflucloxacillinphenoxymethylpenicillinbenzylpenicillinpiperacillin-tazobactam

Members with a page on this site are linked.

Conditions treated

PneumoniaUrinary tract infectionSepsisPeptic ulcerChronic obstructive pulmonary disease (COPD)

Class effects

  • Nausea, diarrhoea, thrush
  • Rash; anaphylaxis in true allergy (rare)
  • Clostridioides difficile colitis with broad-spectrum use

Cautions

  • Most reported penicillin allergies are not true allergy and can be assessed; true anaphylaxis excludes the whole class
  • Amoxicillin causes a rash in glandular fever
  • Every unnecessary course drives resistance

Class warnings

  • Anaphylaxis in true penicillin allergy is the class hazard; a previous immediate reaction to any penicillin is a contraindication to all of them. Most people labelled penicillin-allergic are not, and formal delabelling widens their antibiotic options. UK
  • Antibiotic-associated colitis (C. difficile) can follow any course; severe or persistent diarrhoea needs assessment. UK

Class interactions

WithEffectSource
WarfarinINR may rise; extra checks during and after the course. UK
MethotrexateReduced excretion of methotrexate and increased toxicity. UK
Oral typhoid vaccineInactivated by antibacterials; avoided for 3 days before and after. UK
Allopurinol (with amoxicillin, ampicillin)Increased incidence of rash. UK

Open the Drug Interaction Checker

Educational content. A drug class page describes what a family of medicines has in common. Individual medicines differ in dose, licensing and interactions; treatment choices belong to a prescriber.