💊 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

BNF: Amoxicillin
NICE Antimicrobial Prescribing Guidelines

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Amoxicillin

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