What is the proper use and dosage of Amoxicillin (amoxicillin) for treating bacterial infections?

Amoxicillin: Proper Use and Dosage for Bacterial Infections

Recommended Dosing by Infection Type and Patient Age

For most bacterial respiratory tract infections in adults, amoxicillin 500 mg every 8 hours or 875 mg every 12 hours is the standard dose, with higher doses (up to 4 g/day divided into 2 doses) recommended when penicillin-resistant Streptococcus pneumoniae is suspected. 1

Adult Dosing

Standard Infections:

• Mild to moderate ear/nose/throat, skin, or genitourinary infections: 500 mg every 12 hours OR 250 mg every 8 hours 1
• Severe infections or lower respiratory tract infections: 875 mg every 12 hours OR 500 mg every 8 hours 1
• High-dose regimen for resistant S. pneumoniae: 1.5 to 4 g/day divided into 2-3 doses, with the highest dose (4 g/day in 2 doses) for drug-resistant strains 2, 3

Special Indications:

• H. pylori infection (triple therapy): 1 gram twice daily with clarithromycin 500 mg and lansoprazole 30 mg for 14 days 1
• H. pylori infection (dual therapy): 1 gram three times daily with lansoprazole 30 mg for 14 days 1
• Erysipeloid: 500 mg four times daily for 7-10 days 3

Pediatric Dosing (≥3 Months and <40 kg)

Standard-Dose Regimen:

• Mild to moderate infections: 25 mg/kg/day divided every 12 hours OR 20 mg/kg/day divided every 8 hours 1
• Severe infections or lower respiratory tract infections: 45 mg/kg/day divided every 12 hours OR 40 mg/kg/day divided every 8 hours 1

High-Dose Regimen:

• For resistant pathogens or specific risk factors: 80-100 mg/kg/day divided into 2-3 doses (maximum 4000 mg/day) 3, 4, 2
• This higher dose is critical in areas with >10% prevalence of penicillin-resistant S. pneumoniae 4, 2

Research supports that 40 mg/kg/day amoxicillin achieves significantly better clinical cure (87.9% vs 70.9%) and bacteriologic cure (79.3% vs 54.5%) compared to lower-dose penicillin V for streptococcal pharyngitis, suggesting that inadequate dosing may explain perceived declining penicillin effectiveness. 5

Infants <3 Months

• Maximum dose: 30 mg/kg/day divided every 12 hours due to immature renal function 1

When to Use Amoxicillin-Clavulanate Instead

Amoxicillin-clavulanate should replace amoxicillin alone when β-lactamase-producing organisms (H. influenzae, M. catarrhalis) are suspected or when initial amoxicillin therapy has failed. 3, 4

Key indications for amoxicillin-clavulanate:

• Recent antibiotic use within 4-6 weeks 6, 2
• Daycare attendance 4, 6
• Age <2 years with moderate-severe illness 4
• Incomplete H. influenzae type b vaccination (<3 injections) 3, 4
• Treatment failure with amoxicillin alone 3, 6
• Animal or human bite wounds 3

Adult dosing: 875/125 mg twice daily OR 500/125 mg three times daily 3, 6

Pediatric high-dose: 90 mg/kg/day amoxicillin with 6.4 mg/kg/day clavulanate in 2 divided doses (14:1 ratio formulation causes less diarrhea) 4, 6

Duration of Therapy

Treatment duration varies by infection type:

• Streptococcal pharyngitis: Minimum 10 days to prevent acute rheumatic fever 1
• Community-acquired pneumonia: 10 days for bacterial pneumonia 3, 4
• Acute bacterial sinusitis: 5-7 days in adults, 10-14 days in children 6, 2
• Most other infections: Continue for 48-72 hours after symptom resolution or bacterial eradication 1

A 6-day course of amoxicillin 50 mg/kg/day showed non-inferior efficacy to 10-day penicillin V for streptococcal pharyngitis (83.7% vs 85.3% eradication), though 10 days remains standard to prevent rheumatic fever. 7

Administration and Compliance

Take amoxicillin at the start of meals to minimize gastrointestinal intolerance. 1

For oral suspension:

• Shake well before each use 1
• Discard unused portion after 14 days 1
• May mix with formula, milk, juice, or water and consume immediately 1
• Refrigeration is preferable but not required 1

Once-daily dosing (750-1000 mg) is non-inferior to twice-daily dosing for streptococcal pharyngitis with comparable adverse events and may improve compliance, though twice-daily remains standard. 8

Dosing in Renal Impairment

Patients with severe renal impairment require dose reduction:

• GFR 10-30 mL/min: 500 mg or 250 mg every 12 hours 1
• GFR <10 mL/min: 500 mg or 250 mg every 24 hours 1
• Hemodialysis: 500 mg or 250 mg every 24 hours, with additional dose during and after dialysis 1
• Do NOT use 875 mg dose if GFR <30 mL/min 1

Critical Pitfalls to Avoid

Underdosing is a major cause of treatment failure. The British Thoracic Society doubled its recommended amoxicillin dose to 500-1000 mg every 8 hours due to rising penicillin resistance, aligning with dosing in countries with historically high resistance rates. 3 Using standard doses when high-dose therapy is indicated leads to inevitable treatment failure with resistant organisms and promotes antimicrobial resistance. 4

Most upper respiratory tract infections are viral and do not require antibiotics. Before prescribing amoxicillin for URTIs, ensure the patient meets criteria for bacterial infection (persistent symptoms >10 days, severe symptoms, or "double sickening"). 4 Prescribing antibiotics for viral URTIs increases adverse effects without benefit. 4

Verify suspension concentration (125 mg/5 mL vs 250 mg/5 mL) before calculating volume to avoid dosing errors. 4