Reviewed by MedNext Clinical Team
Amoxicillin is a broad-spectrum aminopenicillin antibiotic and one of the most prescribed drugs in primary and secondary care globally. Its bactericidal activity, oral bioavailability, generally favourable tolerability, and low cost have secured its place as a first-line agent for a wide range of common infections. However, rising resistance rates and appropriate stewardship concerns make rational prescribing increasingly important [1].
Spectrum of Activity
Amoxicillin acts by binding to penicillin-binding proteins (PBPs), inhibiting bacterial cell wall synthesis and causing cell lysis. It is active against a broad range of Gram-positive organisms — including Streptococcus pneumoniae, Streptococcus pyogenes, and Enterococcus faecalis — and selected Gram-negative organisms including Haemophilus influenzae, Escherichia coli, and Helicobacter pylori. It is not active against beta-lactamase-producing organisms; co-amoxiclav (amoxicillin + clavulanate) extends cover to these strains.
Common Clinical Indications
Respiratory Tract Infections
Amoxicillin is a first-line oral antibiotic for community-acquired pneumonia in adults without comorbidities and in children. It is appropriate for acute otitis media and acute bacterial sinusitis where antibiotic therapy is indicated. For uncomplicated upper respiratory tract infections — the vast majority of sore throats and coughs — antibiotics including amoxicillin are not indicated and should not be prescribed.
Urinary Tract Infections
Local resistance patterns should guide UTI prescribing. In many regions, resistance rates of E. coli to amoxicillin exceed 40–50%, making it a poor empirical choice for uncomplicated lower UTI without culture guidance [1]. Where resistance data support its use, amoxicillin 500 mg three times daily for 3–7 days may be appropriate.
Other Indications
Amoxicillin is used in H. pylori eradication regimens (triple or quadruple therapy), Lyme disease (early disseminated disease), dental infections and prophylaxis, and as part of treatment for Listeria monocytogenes in at-risk populations.
Standard Dosing
Adult doses typically range from 250–500 mg three times daily for mild-to-moderate infections, increased to 500 mg–1 g three times daily for severe infections or pneumonia. Paediatric dosing is weight-based (25–50 mg/kg/day divided three times daily). Dose adjustment is required in severe renal impairment (eGFR below 30 mL/min/1.73m²).
Resistance Patterns
Beta-lactamase production is the primary resistance mechanism for amoxicillin. Beta-lactamase-producing strains of H. influenzae, M. catarrhalis, and E. coli are now common, rendering amoxicillin ineffective against them. PBP mutations (as seen in penicillin-resistant S. pneumoniae) provide a second resistance mechanism independent of beta-lactamase production. Prescribers should consult local microbiology resistance data before selecting empirical amoxicillin therapy, particularly for urinary and respiratory infections [1].
Adverse Effects
The most common adverse effects are gastrointestinal — nausea, vomiting, and diarrhoea — and occur in up to 10% of patients. Hypersensitivity reactions range from a benign maculopapular rash (particularly common in patients with infectious mononucleosis) to urticaria, angioedema, and anaphylaxis. A reported penicillin allergy label should always be assessed critically; the majority of patients labelled as penicillin-allergic are not truly allergic on formal evaluation.
References
- Gillies M, Ranakusuma A, Hoffmann T, et al. Common harms from amoxicillin: a systematic review and meta-analysis of randomized placebo-controlled trials. CMAJ. 2015;187(1):E21-31.