Amoxicillin

Generic Name

Amoxicillin

Brand Names

Amoxil

Trimox

Moxatag

Larotid (pediatric suspension)

Biomox (veterinary)

Drug Class

Beta-lactam antibiotic

Aminopenicillin subgroup

Bactericidal antimicrobial agent

Mechanism of Action

Amoxicillin works by inhibiting bacterial cell wall synthesis

It binds to penicillin-binding proteins (PBPs) located inside the bacterial cell wall, interfering with the final stage of peptidoglycan synthesis, a vital structural component of the cell wall

This leads to weakening of the cell wall and subsequent bacterial cell lysis and death

It is effective against a range of gram-positive and some gram-negative bacteria, especially when used with a beta-lactamase inhibitor such as clavulanic acid to overcome resistance

Spectrum of Activity

Susceptible Organisms

Streptococcus pneumoniae

Streptococcus pyogenes

Enterococcus faecalis

Listeria monocytogenes

Neisseria meningitidis

Haemophilus influenzae (non-beta-lactamase producing)

Escherichia coli (some strains)

Proteus mirabilis

Helicobacter pylori

Borrelia burgdorferi

Actinomyces spp

Pasteurella multocida

Resistant Organisms

Beta-lactamase-producing strains of Haemophilus influenzae, Escherichia coli, Klebsiella spp

Pseudomonas aeruginosa

Enterobacter spp

Staphylococcus aureus (especially MRSA)

Indications

Infectious Disease Uses

Acute otitis media

Acute bacterial sinusitis

Streptococcal pharyngitis

Community-acquired pneumonia

Urinary tract infections

Skin and soft tissue infections

Helicobacter pylori eradication (as part of triple therapy)

Lyme disease (early-stage)

Prophylaxis for endocarditis in certain high-risk patients undergoing dental procedures

Dental abscess

Bronchitis and COPD exacerbations

Tonsillitis

Gonorrhea (alternative regimens)

Anthrax exposure (off-label)

Pediatric Use

Highly effective for middle ear infections and streptococcal throat infections in children

Often preferred due to palatability of suspension formulations and well-established safety profile

Dosage and Administration

Adults

250 mg to 500 mg every 8 hours or

500 mg to 875 mg every 12 hours depending on severity of infection

High-dose regimens: up to 1 g every 8 hours for serious infections or resistant Streptococcus pneumoniae

Extended-release tablets: 775 mg once daily for pharyngitis/tonsillitis (Moxatag)

Pediatrics

20–40 mg/kg/day divided every 8 hours or

25–45 mg/kg/day divided every 12 hours

High-dose regimens (e.g., 80–90 mg/kg/day) used in otitis media or penicillin-resistant Streptococcus pneumoniae

Helicobacter pylori eradication

1 g twice daily with clarithromycin and a proton pump inhibitor for 7–14 days

Endocarditis prophylaxis (dental)

2 g orally 30–60 minutes before procedure (adults)

50 mg/kg for children

Renal Impairment

Dose adjustment required in severe renal dysfunction

Hemodialysis patients require supplemental dose post-dialysis

Administration Notes

May be taken with or without food

Suspension should be well shaken and refrigerated if required

Extended-release tablets should not be chewed or crushed

Contraindications

Hypersensitivity to amoxicillin, penicillins, or any component of the formulation

History of serious allergic reaction to beta-lactams (e.g., anaphylaxis, Stevens-Johnson syndrome)

Warnings and Precautions

Hypersensitivity Reactions

Cross-reactivity between penicillins and cephalosporins may occur

Can lead to anaphylaxis, angioedema, urticaria, or severe cutaneous adverse reactions

Clostridioides difficile Infection

Like other antibiotics, amoxicillin may cause C. difficile-associated diarrhea, especially with prolonged use

Renal Impairment

Dose adjustments necessary

Monitor renal function in patients on prolonged therapy

Mononucleosis

Amoxicillin use in Epstein–Barr virus infections may result in a characteristic rash and is discouraged

Seizure Risk

High doses in patients with renal impairment or seizure history may increase risk of CNS effects

Adverse Effects

Common

Diarrhea

Nausea

Vomiting

Rash (especially in patients with mononucleosis)

Headache

Taste disturbance

Less Common

Abdominal pain

Flatulence

Dyspepsia

Fungal overgrowth (oral/vaginal candidiasis)

Serious

Anaphylaxis

Stevens-Johnson syndrome

Toxic epidermal necrolysis

Hepatitis or cholestatic jaundice (rare)

Hemolytic anemia

Thrombocytopenia

Interstitial nephritis

Seizures (at high doses or with renal dysfunction)

Drug Interactions

Oral contraceptives

Effectiveness may be reduced by alterations in gut flora (though data is controversial, backup contraception often advised)

Allopurinol

Co-administration increases risk of rash

Anticoagulants (warfarin)

May enhance anticoagulant effect by altering gut flora and vitamin K production

Methotrexate

Amoxicillin may decrease renal clearance of methotrexate, increasing its toxicity

Live vaccines (oral typhoid)

Antibiotics may reduce effectiveness

Probenecid

Inhibits renal excretion of amoxicillin, increasing plasma concentrations and half-life

Use in Special Populations

Pregnancy

Category B

Widely considered safe and used commonly in all trimesters

Lactation

Excreted in breast milk in small amounts

Generally considered safe for breastfeeding; monitor for gastrointestinal disturbance in infant

Pediatrics

Extensively used with good safety profile

Formulated as palatable suspensions with various flavors

Elderly

No specific precautions, but renal function monitoring is advised

Use in Dentistry

Frequently prescribed for dental infections, abscesses, and periodontal diseases

Amoxicillin is first-line prophylaxis against infective endocarditis in at-risk patients undergoing invasive dental procedures

Use in Surgery

Used for prophylaxis in selected surgeries involving the head and neck or gastrointestinal tract

Overdose

Symptoms include nausea, vomiting, diarrhea, crystalluria, renal dysfunction

Treatment is supportive

Hemodialysis may be used to remove the drug in cases of significant overdose or renal failure

Formulations

Tablets: 250 mg, 500 mg, 875 mg

Capsules: 250 mg, 500 mg

Suspension: 125 mg/5 mL, 250 mg/5 mL, 400 mg/5 mL

Chewable tablets: 125 mg, 250 mg

Extended-release tablets: 775 mg

Injectable form: Not available for amoxicillin alone (amoxicillin-clavulanic acid is available for IV use)

Storage

Tablets and capsules: store at room temperature

Reconstituted suspension: refrigerate and use within 14 days

Do not freeze suspension

Antibiotic Stewardship Considerations

Avoid use in viral infections such as the common cold or influenza

Use culture and sensitivity results to guide therapy

Reserve high-dose regimens for specific indications such as otitis media with resistant Streptococcus pneumoniae

Educate patients to complete the full course of therapy even if symptoms improve

Resistance Considerations

Resistance primarily due to beta-lactamase production in organisms like Haemophilus influenzae and E. coli

Co-formulation with clavulanic acid (as in co-amoxiclav) inhibits beta-lactamases and extends spectrum

Not effective against MRSA or Pseudomonas aeruginosa