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Amoxicillin

Amoxicillin

Amoxil Moxatag Trimox Larotid

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Updated: December 06, 2025

Overview

Amoxicillin is a broad-spectrum, bactericidal antibiotic belonging to the aminopenicillin class of beta-lactam antibiotics. It is one of the most commonly prescribed antibiotics worldwide due to its effectiveness against a wide range of bacterial pathogens and favorable safety profile. Amoxicillin works by inhibiting bacterial cell wall synthesis, making it particularly effective against actively dividing organisms. It demonstrates good oral bioavailability and tissue penetration, including respiratory tract secretions, middle ear fluid, and urine. The drug is often used as first-line therapy for various community-acquired infections.

Drug Class

Antibacterial, Aminopenicillin, Beta-lactam antibiotic, Penicillin-class antimicrobial

Mechanism of Action

Binds to penicillin-binding proteins (PBPs) on bacterial cell walls Inhibits transpeptidation reaction in peptidoglycan synthesis Disrupts bacterial cell wall integrity Causes osmotic instability and bacterial cell lysis Effective against gram-positive and some gram-negative organisms

Pharmacokinetics

Absorption: Well-absorbed orally (74-92% bioavailability), not significantly affected by food Distribution: Widely distributed (Vd 0.3-0.4 L/kg), penetrates into middle ear fluid, bronchial secretions, and bone Metabolism: Minimal hepatic metabolism (<10%) Excretion: Primarily renal (60-80% unchanged in urine) Half-life: 1-1.5 hours in normal renal function (prolonged in renal impairment)

Pharmacodynamics

Time-dependent bactericidal activity Demonstrates post-antibiotic effect against some organisms Minimum inhibitory concentration (MIC) dependent Effective against susceptible organisms during active growth phase

Indications

• Acute otitis media: First-line treatment for bacterial middle ear infections • Streptococcal pharyngitis: Treatment of Group A Streptococcus infections • Community-acquired pneumonia: For susceptible bacterial causes • Urinary tract infections: Uncomplicated cystitis and pyelonephritis • Skin/skin structure infections: Cellulitis, erysipelas • Helicobacter pylori eradication: Used in combination therapy • Lyme disease: Early stage treatment in children

Contraindications

Absolute: • Hypersensitivity to amoxicillin or other beta-lactam antibiotics • History of amoxicillin-associated cholestatic jaundice Relative: • Infectious mononucleosis (high rash risk) • Severe renal impairment (requires dose adjustment) • History of GI disease (especially colitis)

Dosage & Administration

Adults (general infections): 250-500 mg every 8 hours or 500-875 mg every 12 hours Children (>3 months): 20-45 mg/kg/day divided every 8-12 hours Acute otitis media: 80-90 mg/kg/day divided every 12 hours (high-dose) H. pylori eradication: 1 g twice daily with other agents Renal impairment (CrCl <30 mL/min): Extended dosing interval to every 12-24 hours

Special Populations

Pediatric: Safe for children >3 months, weight-based dosing required Geriatric: Consider renal function for dosing, increased risk of CDAD Renal impairment: Requires dose adjustment (CrCl <30 mL/min) Hepatic impairment: No significant adjustment needed

Adverse Effects

Common (>10%): • Diarrhea (7-14%) • Nausea (2-10%) • Skin rash (3-10%) Serious (<1%): • Anaphylaxis • Clostridioides difficile-associated diarrhea (CDAD) • Stevens-Johnson syndrome • Drug-induced liver injury • Antibiotic-associated colitis

Drug Interactions

• Probenecid: Increases amoxicillin serum concentrations • Oral contraceptives: May reduce contraceptive effectiveness • Warfarin: May potentiate anticoagulant effect • Allopurinol: Increased risk of skin rash • Methotrexate: May increase methotrexate toxicity

Warnings & Precautions

• Serious hypersensitivity reactions including anaphylaxis • Risk of CDAD (may occur months after therapy) • Potential for superinfections with prolonged use • Increased rash incidence in patients with EBV infection • Possible false-positive urine glucose tests • May cause tooth discoloration with prolonged use

Pregnancy & Lactation

Pregnancy: FDA Category B - Generally considered safe, extensive clinical use without apparent harm Lactation: Excreted in breast milk (0.01-0.8% maternal dose), considered compatible with breastfeeding

Monitoring Parameters

• Clinical signs of infection resolution • Renal function in prolonged therapy • Signs of hypersensitivity reactions • Stool characteristics for CDAD • Liver function tests in prolonged use • CBC with extended therapy

Patient Counseling

• Complete full course of therapy even if symptoms improve • Report any rash, diarrhea, or unusual symptoms • May take with food to reduce GI upset • Do not share medication with others • Inform provider of any allergy history • Store suspension in refrigerator (discard after 14 days)

Storage & Stability

Tablets: Store at 20-25°C (68-77°F) Oral suspension: Refrigerate (2-8°C/36-46°F) Stable for 7-14 days after reconstitution depending on formulation Protect from moisture

Clinical Pearls

• First-line choice for acute otitis media in children • High-dose regimens (80-90 mg/kg/day) for penicillin-resistant pneumococci • Always consider local resistance patterns • Not effective against beta-lactamase producing organisms • Rash development doesn't always indicate true penicillin allergy • Consider adding beta-lactamase inhibitor (e.g., clavulanate) for broader coverage

References

• UpToDate: Amoxicillin drug information • Goodman & Gilman's: The Pharmacological Basis of Therapeutics, 13th ed. • FDA Prescribing Information: Amoxil (amoxicillin) • The Sanford Guide to Antimicrobial Therapy • American Academy of Pediatrics Red Book