Amoxicillin

Amoxicillin is indicated for infections caused by susceptible Gram-positive and Gram-negative bacteria, including Streptococcus pneumoniae, Haemophilus influenzae, Escherichia coli, and Listeria monocytogenes. It is a first-line treatment for acute otitis media in children, streptococcal pharyngitis, and community-acquired pneumonia. It is also used for Helicobacter pylori eradication in combination with clarithromycin and a proton-pump inhibitor, and as prophylaxis for infective endocarditis in certain dental procedures. The drug is effective against Borrelia burgdorferi, making it an option for early Lyme disease, and is used in anthrax post-exposure prophylaxis. Amoxicillin is not effective against viral infections or β-lactamase-producing organisms such as Staphylococcus aureus (unless combined with clavulanate).

Common adverse effects include diarrhea, nausea, rash, and vaginal candidiasis. A non-allergic maculopapular rash occurs in about 3–10% of patients, particularly those with Epstein-Barr virus infection. Serious reactions include anaphylaxis, Stevens-Johnson syndrome, and antibiotic-associated colitis caused by Clostridioides difficile. Amoxicillin is contraindicated in individuals with a history of immediate hypersensitivity to penicillins. It is generally safe in pregnancy (FDA category B) and is excreted in breast milk in small amounts. Renal dose adjustment is required in severe impairment. The drug can also cause crystalluria at high doses, and rare cases of drug-induced hepatitis have been reported.

Amoxicillin is a β-lactam antibiotic that binds to penicillin-binding proteins (PBPs) in the bacterial cell wall, inhibiting transpeptidation and activating autolytic enzymes, leading to cell death. It has a broader spectrum than penicillin G, including activity against some Gram-negative bacilli. It is acid-stable and well absorbed orally, with a bioavailability of about 80%, and food does not significantly reduce absorption. The drug is excreted primarily by the kidneys. Bacterial resistance arises mainly through β-lactamase production, alteration of PBPs, or efflux pumps. To counter β-lactamase-mediated resistance, amoxicillin is combined with clavulanic acid (co-amoxiclav), which extends its activity to include β-lactamase-producing strains of Staphylococcus aureus and Haemophilus influenzae.

Amoxicillin was developed by British scientists and introduced in 1972, following the earlier discovery of ampicillin. It is one of the most prescribed antibiotics worldwide and is on the WHO Model List of Essential Medicines. A notable niche application is its use in treating Lyme disease in children and pregnant women, where doxycycline is contraindicated. Amoxicillin has also been studied for its potential anti-inflammatory effects independent of its antibacterial action. In veterinary medicine, it is used in livestock and pets, contributing to concerns about antimicrobial resistance. The drug is also used in some research settings to create animal models of gut dysbiosis. Additionally, amoxicillin is a common cause of drug-induced rash in patients with infectious mononucleosis, a phenomenon first described in the 1960s.

β-lactam antibiotic

A class of antibiotics containing a β-lactam ring that inhibits bacterial cell wall synthesis.

Clavulanic acid

A β-lactamase inhibitor that protects amoxicillin from degradation by bacterial enzymes.

Penicillin-binding proteins

Enzymes involved in bacterial cell wall synthesis that are targets of β-lactam antibiotics.

Otitis media

Inflammation of the middle ear, often bacterial in origin.

Stevens-Johnson syndrome

A severe, potentially fatal skin reaction that can be caused by antibiotics.