amoxicillin
Penicillin antibiotics· Brands: Amoxil, Moxatag, Trimox, Wymox, Dispermox, NovamoxinIMIVPO
Mechanism of action
Aminopenicillin; cell wall synthesis inhibition.
Pharmacokinetics
Amoxicillin is well absorbed from the gastrointestinal tract and is acid-stable, with oral bioavailability approximately 90%. It penetrates widely into body tissues and fluids, including tonsils, sinuses, sputum, and bone, and it crosses the placenta. It has minimal hepatic metabolism, and around 60% to 70% of the drug is excreted unchanged in the urine via glomerular filtration and tubular secretion, with active tubular secretion being the primary route. The elimination half-life is about 1 hour, but it is prolonged in renal impairment, requiring dose adjustments in patients with creatinine clearance below 30 mL/min. It is also removed by haemodialysis.
Indications & dosing
Community-acquired pneumonia (mild)
- adult · PO: 500–1000 mg TDS
Contraindications & precautions
- Penicillin allergy
- Adjust dose in renal impairment; accumulate in severe renal failure.
- Use with caution in patients with history of allergy to cephalosporins or other beta-lactams.
- Monitor for signs of superinfection, especially with prolonged use.
- Consider the risk of rash in patients with infectious mononucleosis or concurrent allopurinol use.
Adverse effects
- Rash / hypersensitivityserious
Interactions
- warfarinmoderateMonitor INR closely, particularly within the first week of concurrent therapy. Consider more frequent INR checks and adjust warfarin dose as needed. Advise patient to report signs of bleeding.
- methotrexatemajorAvoid concurrent use if possible. If unavoidable, use with caution and monitor serum methotrexate concentrations, full blood count, and renal function. Consider reducing the methotrexate dose and providing appropriate rescue with folinic acid. An alternative antibiotic should be considered in high-dose methotrexate regimens.
Renal & hepatic adjustment
- eGFR 30+: No dose adjustment required.
- eGFR 10–30: Reduce dose or increase dosing interval. Consider 250-500 mg every 12 hours.
- eGFR 0–10: Reduce dose or increase dosing interval. Consider 250-500 mg every 24 hours.
Monitoring, pregnancy & toxicity
- Renal function · In prolonged therapy or renal impairment
- Blood counts · Rarely, with prolonged therapy
- Signs of allergic reaction
- Pregnancy: Amoxicillin is generally considered safe in pregnancy. Use when clearly indicated and weigh benefits against risks.
- Toxicity: Gastrointestinal symptoms (nausea, vomiting, diarrhoea) · Supportive care and adequate hydration.
- Toxicity: Allergic reactions (rash, urticaria, anaphylaxis) · Stop the drug and treat with antihistamines or epinephrine as appropriate.
- Toxicity: Rare crystaluria with high doses · Maintain high fluid intake and monitor renal function.
Clinical notes
Amoxicillin is indicated for the treatment of respiratory tract infections, otitis media, sinusitis, urinary tract infections, skin and soft tissue infections, and eradication of Helicobacter pylori in combination with other agents. It is also used for the prophylaxis of infective endocarditis. Before prescribing, always check for penicillin allergy; cross-reactivity with cephalosporins may occur. Common adverse effects include diarrhoea, nausea, vomiting, and skin rash. Use with caution in patients with renal impairment or infectious mononucleosis, as such patients are more prone to rash. Consider combination with clavulanate if beta-lactamase-producing organisms are suspected.
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BNF; local formulary