Amoxicillin
Broad-spectrum aminopenicillin antibiotic for bacterial infections
Amoxicillin is an aminopenicillin antibiotic with broad-spectrum bactericidal activity against many gram-positive and gram-negative organisms. It was developed in the early 1970s as an improvement over ampicillin and is now one of the most frequently prescribed antibiotics worldwide. Amoxicillin is distinguished from older penicillins by better oral bioavailability and a broader antibacterial spectrum.
Amoxicillin is available as capsules, dispersible tablets, oral suspensions, and injectable preparations. It is also combined with the beta-lactamase inhibitor clavulanic acid (co-amoxiclav) to extend coverage against beta-lactamase-producing bacteria. In Germany, amoxicillin is among the most used antibiotics in primary care for respiratory and urinary tract infections.
Mechanism of Action
Amoxicillin belongs to the beta-lactam class of antibiotics. It inhibits bacterial cell wall synthesis by binding covalently to penicillin-binding proteins (PBPs), which are enzymes responsible for the final steps in peptidoglycan cross-linking. Without intact peptidoglycan, the bacterial cell wall loses structural integrity and the bacterium lyses. Amoxicillin is bactericidal against susceptible organisms and is more effective in a time-dependent manner — efficacy correlates with the proportion of time that drug concentrations exceed the minimum inhibitory concentration (MIC) during the dosing interval.
The main limitation is susceptibility to beta-lactamases produced by many resistant bacteria including Staphylococcus aureus, Haemophilus influenzae, Moraxella catarrhalis, and Escherichia coli. Co-administration with clavulanic acid overcomes this resistance mechanism for most clinically relevant beta-lactamase producers.
Indications
- Upper respiratory tract infections: Streptococcal tonsillitis/pharyngitis, acute bacterial sinusitis, acute otitis media
- Lower respiratory tract infections: Community-acquired pneumonia (in combination with a macrolide when atypicals are possible), acute exacerbation of chronic bronchitis
- Urinary tract infections: Uncomplicated cystitis caused by susceptible organisms; pyelonephritis (usually requires combination or more active agents)
- Helicobacter pylori eradication: As part of triple or quadruple therapy with proton pump inhibitors and clarithromycin or metronidazole
- Skin and soft tissue infections: Caused by susceptible streptococci; erysipelas
- Endocarditis prophylaxis: In at-risk patients undergoing dental procedures
- Lyme disease (early stage): Oral treatment of erythema migrans
Dosage and Administration
Adults (standard infections): 500 mg three times daily or 875 mg twice daily (amoxicillin/clavulanate formulation). Severe infections: Up to 1000 mg three times daily. H. pylori eradication: 1000 mg twice daily as part of a combination regimen for 7 to 14 days. Children: 25 to 50 mg per kg body weight per day in two to three divided doses depending on the severity and type of infection.
Amoxicillin can be taken with or without food. The oral suspension is particularly suitable for children and patients with swallowing difficulties. Dose reduction is required in patients with severe renal impairment (GFR below 30 ml/min). Treatment duration for common respiratory infections is typically 5 to 7 days; for H. pylori 7 to 14 days; for skin infections up to 14 days.
Side Effects
Common (1 to 10 percent): Diarrhoea, nausea, vomiting, skin rash. The rash associated with amoxicillin is typically a maculopapular (non-urticarial) exanthema, often appearing after 5 to 10 days, particularly in patients with infectious mononucleosis (EBV infection) — this is not a true penicillin allergy but a drug-virus interaction.
Occasional: Abdominal discomfort, flatulence, candidal superinfection (oral thrush, vaginal candidiasis).
Rare but serious: Anaphylaxis and severe hypersensitivity reactions (urticaria, angioedema, anaphylactic shock) — require immediate cessation and emergency treatment. Clostridium difficile-associated diarrhoea (antibiotic-associated colitis). Hepatic dysfunction including cholestatic jaundice (more common with co-amoxiclav than amoxicillin alone). Stevens-Johnson syndrome and toxic epidermal necrolysis are very rare.
Interactions
- Anticoagulants (warfarin, phenprocoumon): Amoxicillin can affect gut flora and vitamin K production, potentially enhancing anticoagulant effect; INR monitoring recommended
- Methotrexate: Amoxicillin reduces renal tubular secretion of methotrexate, increasing toxicity risk; close monitoring required
- Oral contraceptives: Theoretical reduction in efficacy through gut flora changes; clinical significance debated; additional contraceptive measures advisable during and for 7 days after antibiotic therapy
- Probenecid: Inhibits renal tubular secretion of amoxicillin, increasing and prolonging plasma levels; used therapeutically in some penicillin regimens
- Allopurinol: Increases the frequency of skin rash when combined with amoxicillin
Special Notes
Penicillin allergy: Before prescribing amoxicillin, always clarify the patient's allergy history. True IgE-mediated penicillin allergy is a contraindication. Many patients with a self-reported penicillin allergy are actually tolerant on formal allergy testing; nevertheless, caution is required. Patients with confirmed penicillin allergy may also react to cephalosporins (cross-reactivity approximately 1 to 2 percent) though this risk is lower than historically thought.
Antibiotic stewardship: Amoxicillin should only be used for confirmed or highly suspected bacterial infections. Viral respiratory infections such as the common cold and influenza do not respond to amoxicillin and its use in these conditions contributes to antibiotic resistance.
Pregnancy and breastfeeding: Amoxicillin is considered safe in all trimesters of pregnancy and during breastfeeding and is among the preferred antibiotics for bacterial infections in these periods.
Related Topics
- Co-amoxiclav (Amoxicillin/Clavulanate) — Extended spectrum combination for resistant organisms
- Clarithromycin — Macrolide antibiotic for respiratory infections
- Doxycycline — Tetracycline antibiotic, alternative in penicillin allergy
- All active ingredients overview
Frequently Asked Questions
Why does amoxicillin cause a rash in glandular fever?
In patients with Epstein-Barr virus infection (infectious mononucleosis or glandular fever), amoxicillin almost always triggers a widespread maculopapular rash. This is a drug-virus interaction, not a true penicillin allergy. After recovery from the EBV infection, amoxicillin can usually be used again without problem. Allergy testing should be performed before labelling the patient as penicillin-allergic.
What is the difference between amoxicillin and co-amoxiclav?
Co-amoxiclav combines amoxicillin with clavulanic acid, a beta-lactamase inhibitor. This combination covers bacteria that would be resistant to amoxicillin alone due to beta-lactamase production, such as many strains of Haemophilus influenzae, Moraxella catarrhalis, and Staphylococcus aureus. Co-amoxiclav has a slightly higher rate of gastrointestinal side effects.
Should amoxicillin be taken with food?
Amoxicillin can be taken with or without food. Unlike some other antibiotics, food does not significantly affect absorption. However, taking amoxicillin with food may reduce gastrointestinal side effects such as nausea.