Amoxicillin !
A well-absorbed oral penicillin for ear, sinus and chest infections; combined with other drugs it clears H. pylori from the stomach.
Updated 2026-11-01
The figure shows the transpeptidase domain of PBP3 from Chlamydia trachomatis with amoxicillin bound; it stands for the PBPs of the bacteria amoxicillin treats.
Mechanism of action
Amoxicillin is an aminopenicillin: it acylates the PBPs and stops peptidoglycan cross-linking, like penicillin G. Its amino group widens the spectrum to some gram-negative bacteria, and it is stable in gastric acid, so it is well absorbed by mouth. β-lactamases destroy it, which is why it is often paired with clavulanic acid.
- Absorbed from the gut and reaches the infected tissue
- Binds and acylates the bacterial PBPs
- Peptidoglycan cross-links stop forming
- The wall fails and the bacterium lyses
Pharmacokinetics
- Absorption
- Stable in gastric acid and rapidly absorbed by mouth, with or without food.
- Elimination
- Mostly unchanged in the urine; probenecid delays it.
- Half-life
- About 1 hour.
Indications
- Acute otitis media. Acute otitis media →
- Acute bacterial sinusitis (alone or with clavulanic acid). Acute rhinosinusitis →
- Lower respiratory tract infections such as community-acquired pneumonia. Community-acquired pneumonia →
- H. pylori eradication, together with a proton pump inhibitor and clarithromycin. Helicobacter pylori gastritis →
- Group A streptococcal pharyngitis. Streptococcal pharyngitis →
Adverse effects
Common
- Nausea, vomiting and diarrhoea
- Rash
- An erythematous rash in a high share of patients with infectious mononucleosis (Epstein–Barr virus) who receive an aminopenicillin
Contraindications
- History of allergy to penicillins
- Suspected infectious mononucleosis (rash is very likely)
Key interactions
- Probenecid
- Reduces tubular secretion: higher and longer-lasting amoxicillin levels.
- Tetracyclines, macrolides, chloramphenicol and sulfonamides
- May reduce the bactericidal effect.
- Combined oral contraceptives
- Amoxicillin may make them less effective.
What is monitored
- Clinical response in the first days
- Liver tests if jaundice appears with amoxicillin–clavulanic acid
Test yourself
Think of the answer, then tap the question to see it.
A teenager with sore throat and large lymph nodes gets a widespread rash after starting amoxicillin. What is the likely cause?
Infectious mononucleosis (Epstein–Barr virus). The rash is a typical non-allergic reaction to aminopenicillins in EBV infection, and the antibiotic was not needed for a viral illness.
Why can amoxicillin be given by mouth, unlike penicillin G?
It is stable in gastric acid and rapidly absorbed from the gut, with or without food.
Why is amoxicillin often combined with clavulanic acid?
Clavulanic acid inhibits many β-lactamases, so it protects amoxicillin from bacteria that would otherwise destroy it.
In H. pylori eradication, what are amoxicillin's usual partners?
A proton pump inhibitor, which lowers gastric acid, plus another antibiotic such as clarithromycin.
Related drugs
Related conditions
- Acute otitis media
- Acute rhinosinusitis
- Community-acquired pneumonia
- Helicobacter pylori gastritis
- Streptococcal pharyngitis
← Back to the class: Beta-lactams
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: Amoxicillin: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); DailyMed - amoxicillin- Amoxicillin capsule amoxicillin- Amoxicillin tablet, film coated (DailyMed (U.S. National Library of Medicine), 2026); RCSB PDB - 6I1F: Crystal structure of TP domain from Chlamydia trachomatis Penicillin-Binding Protein 3 in complex with amoxicillin (RCSB Protein Data Bank, 2026); eEML - Electronic Essential Medicines List (World Health Organization, 2025).
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Representative model, for educational purposes only.