Amoxicillin
Amoxicillin is an oral aminopenicillin antibiotic used in adults and children for respiratory, urinary, dental and otologic bacterial infections. It inhibits bacterial cell wall synthesis and is widely available as a first-line option in many indications.
- Chemical formula
- C16H19N3O5S
- CAS number
- 26787-78-0
- ATC code
- J01CA04
- Molecular weight
- 365.40 g/mol
- Drug class
- Aminopenicillin antibiotic
- Also known as
- BRL 2333, Amoxicilina
What is it?
Amoxicillin is a beta-lactam antibiotic of the aminopenicillin subclass developed in the late 1960s and approved in the early 1970s. It is supplied as oral capsules, tablets, dispersible tablets, oral suspension and parenteral preparations, and is widely used in primary care. Amoxicillin appears on the World Health Organization List of Essential Medicines. It is also formulated in fixed-dose combination with the beta-lactamase inhibitor clavulanate (amoxicillin-clavulanate).
Mechanism of action
Amoxicillin inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins, which prevents the cross-linking of peptidoglycan layers and triggers bacterial autolysis. It is bactericidal against susceptible organisms, with broader Gram-positive and Gram-negative coverage than benzylpenicillin. Many bacteria produce beta-lactamase enzymes that hydrolyse amoxicillin; combination with clavulanate, a beta-lactamase inhibitor, restores activity in this setting.
Pharmacokinetics
Amoxicillin is well absorbed orally, with bioavailability of approximately 75% to 90%. Peak plasma concentrations are reached one to two hours after a dose. Plasma protein binding is low (about 17% to 20%). Amoxicillin is excreted predominantly unchanged in urine, and renal impairment requires dose adjustment. The terminal half-life is approximately one hour in adults with normal renal function, supporting two to three daily oral doses depending on the indication.
Indications
Amoxicillin is approved in adults and children for the treatment of bacterial infections of the upper and lower respiratory tract, urinary tract, ear, nose and throat, dental infections, skin and soft tissue infections, and as part of Helicobacter pylori eradication regimens. According to clinical guidelines, it is a first-line option in many of these indications when local resistance allows and is widely used in paediatrics with weight-based dosing.
Safety profile
Amoxicillin is generally well tolerated. The most common adverse effects are gastrointestinal: nausea, diarrhoea and abdominal discomfort. Hypersensitivity reactions, including rash, urticaria and rarely anaphylaxis, are well documented; cross-reactivity with other penicillins and partial cross-reactivity with cephalosporins should be considered. Maculopapular rashes during glandular fever are characteristic. According to the prescribing information, severe hypersensitivity is a contraindication and renal impairment requires dose adjustment.
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Frequently asked questions
How is amoxicillin different from amoxicillin-clavulanate? ▾
Amoxicillin alone is an aminopenicillin that is hydrolysed by many bacterial beta-lactamases. Amoxicillin-clavulanate adds clavulanate, a beta-lactamase inhibitor that protects amoxicillin and broadens activity against beta-lactamase-producing organisms such as Haemophilus influenzae, Moraxella catarrhalis and many anaerobes. According to clinical guidelines, the combination is used for moderate to severe respiratory, urinary and bite-wound infections.
Is amoxicillin safe in penicillin allergy? ▾
No. Patients with confirmed allergy to penicillins should not receive amoxicillin because of the risk of severe hypersensitivity reactions, including anaphylaxis. Cross-reactivity with cephalosporins exists, particularly with first-generation agents and shared side chains. According to clinical guidelines and the prescribing information, the medical history and any prior penicillin reaction must be reviewed by a clinician before any prescription, with formal allergy testing in selected cases.
Why does amoxicillin cause diarrhoea? ▾
Antibiotics including amoxicillin disrupt the normal gut microbiota, which can lead to loose stools or diarrhoea, generally mild and self-limited. In some cases, antibiotic-associated diarrhoea reflects overgrowth of Clostridioides difficile, which can be severe and requires medical assessment. According to clinical guidelines, persistent or bloody diarrhoea during or after antibiotic treatment should prompt consultation with the prescriber.
Should amoxicillin be taken with food? ▾
Amoxicillin can be taken with or without food, although taking it with food may reduce gastrointestinal upset for some patients. According to the prescribing information, regular spacing of doses (every 8 or 12 hours) is more important than food timing for maintaining therapeutic concentrations. Suspension formulations should be shaken before each dose, and the prescribed course should be completed unless otherwise advised.
What are the main contraindications for amoxicillin? ▾
Amoxicillin is contraindicated in known hypersensitivity to penicillins or any beta-lactam antibiotic. Caution is required in severe renal impairment with dose adjustment, in concomitant allopurinol therapy (increased rash incidence), in glandular fever and in pregnancy and breastfeeding (although generally considered acceptable). According to the prescribing information, the medical history must be reviewed by a clinician before any prescription.
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