Amoxicillin 💊
⚠️ Avoid in patients with glandular fever (EBV), CMV infection, ALL/CLL as it can cause a characteristic rash.
📖 About
Always check the BNF link here.
- 💊 Amoxicillin – a broad-spectrum aminopenicillin.
- Better absorbed orally than ampicillin → higher bioavailability and fewer GI side effects.
- Often combined with clavulanic acid (as Co-amoxiclav) to overcome beta-lactamase resistance.
⚡ Mechanism of Action
- 🔪 Beta-lactam antibiotic – interferes with bacterial cell wall synthesis.
- Inhibits cross-linking of peptidoglycan in bacterial cell walls → cell lysis.
- Destroyed by beta-lactamase enzymes (unless combined with clavulanic acid).
🧬 Spectrum of Activity
- ✅ E. coli (some strains), Proteus mirabilis, Shigella, Salmonella.
- ✅ Haemophilus influenzae, some Enterococci.
- ❌ Not reliable against beta-lactamase–producing organisms or Pseudomonas.
🩺 Indications & Typical Adult Dosing
| Indication |
Typical Adult Dose |
| 🌬️ Respiratory infections (e.g. pneumonia) |
500 mg PO every 8 hours, or 875 mg PO every 12 hours |
| 🦠 Mild infections (ENT, skin, urinary) |
250–500 mg PO TDS |
| 🧫 Anthrax (post-exposure prophylaxis) |
500 mg PO every 8 hours |
| 🧠 Listeria meningitis (with another antibiotic) |
2 g IV every 4 hours for 10–14 days |
| ❤️ Endocarditis (with gentamicin or similar) |
2 g IV every 4–6 hours |
📚 Teaching Points
- Amoxicillin is first-line for many community infections in the UK (e.g. pneumonia, otitis, sinusitis).
- High doses (e.g. meningitis, endocarditis) are used for CNS penetration and synergy with aminoglycosides.
- Lower doses may suffice in mild ENT/skin infections, but stewardship principles apply.
- BNF and local antimicrobial guidelines should always be checked before prescribing.
🔄 Interactions
- ⬇️ May reduce efficacy of the oral contraceptive pill (OCP) – advise backup contraception.
- ⬆️ Probenecid increases amoxicillin levels by reducing renal clearance.
- ⬆️ Risk of methotrexate toxicity (reduced clearance).
⚠️ Cautions
- Allergy risk – always ask about penicillin history.
- Characteristic rash if given with glandular fever (EBV), CMV, ALL or CLL.
- Adjust dose in renal impairment.
🚫 Contraindications
- Known penicillin allergy.
💥 Side Effects
- 🫁 Hypersensitivity: rash, urticaria, angioedema, anaphylaxis.
- 🌡️ Red maculopapular rash – especially in EBV, CMV, or haematological malignancy.
- 🤢 GI upset: nausea, vomiting, diarrhoea.
- 💩 Antibiotic-associated colitis (rare).
- 🦠 Opportunistic infection: oral/vaginal candidiasis.
- Rare: interstitial nephritis, haemolytic anaemia, hepatitis.
📑 References
