What is the recommended dosage of Amoxicillin (amoxicillin) for adults with infections?

Amoxicillin Dosage for Adults with Infections

For most adult infections, amoxicillin 500 mg twice daily or 250 mg three times daily is appropriate for mild-to-moderate disease, while 875 mg twice daily or 500 mg three times daily is recommended for severe infections or respiratory tract involvement. 1

Standard Dosing by Infection Severity

Mild-to-Moderate Infections

• Ear/nose/throat, skin/skin structure, and genitourinary tract infections: 500 mg every 12 hours OR 250 mg every 8 hours 1
• The FDA-approved dosing range for adults is 750-1750 mg/day in divided doses every 8-12 hours 1

Severe Infections

• All infection types including lower respiratory tract: 875 mg every 12 hours OR 500 mg every 8 hours 1
• This higher dosing is particularly important for respiratory infections where bacterial resistance is a concern 2

Infection-Specific Recommendations

Acute Bacterial Rhinosinusitis

• Mild disease without recent antibiotic use (past 4-6 weeks): Amoxicillin 1.5-4 g/day in divided doses 2
• Mild disease with recent antibiotic use OR moderate disease: High-dose amoxicillin 4 g/day (maximum 1 g every 12 hours) to overcome penicillin-resistant Streptococcus pneumoniae 2
• The standard 500 mg twice daily provides 87-88% calculated clinical efficacy, while higher doses approach 90-91% efficacy 2

Lower Respiratory Tract Infections

• 875 mg every 12 hours is as effective as 500 mg every 8 hours for lower respiratory tract infections, with the advantage of improved compliance and potentially less diarrhea 3
• Both regimens demonstrated 93-94% clinical success rates in comparative trials 3

Streptococcal Pharyngitis

• Standard treatment: 1000 mg once daily for 10 days 4
• Minimum 10 days of therapy is mandatory for any Streptococcus pyogenes infection to prevent acute rheumatic fever, regardless of clinical improvement 1, 4
• Amoxicillin is first-line therapy due to narrow spectrum, minimal adverse effects, and low cost 4

Helicobacter pylori Infection

• Triple therapy: 1 gram amoxicillin twice daily (every 12 hours) combined with 500 mg clarithromycin and 30 mg lansoprazole for 14 days 1
• Dual therapy: 1 gram amoxicillin three times daily (every 8 hours) with 30 mg lansoprazole for 14 days 1

Critical Dosing Considerations

Administration Timing

• Take at the start of a meal to minimize gastrointestinal intolerance 1
• This timing is particularly important for higher doses to reduce adverse effects

Treatment Duration

• Continue for minimum 48-72 hours beyond symptom resolution or evidence of bacterial eradication 1
• For Streptococcus pyogenes infections, complete the full 10-day course even if symptoms resolve earlier 1, 4
• Some infections may require several weeks of therapy with clinical/bacteriological follow-up for months after cessation 1

Renal Impairment Adjustments

• GFR 10-30 mL/min: 500 mg or 250 mg every 12 hours (depending on infection severity) 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 at end of dialysis 1
• Do NOT use the 875 mg dose in patients with GFR <30 mL/min 1

Common Pitfalls to Avoid

Resistance Considerations

• 25-50% of S. pneumoniae strains show penicillin resistance, which can often be overcome by increasing amoxicillin to 90 mg/kg/day (maximum 1 g every 12 hours in adults) 2
• Recent antibiotic use within 4-6 weeks is a major risk factor for resistant organisms and should prompt higher dosing 2
• Nearly 50% of H. influenzae and 90-100% of M. catarrhalis produce β-lactamase in most geographic areas, making amoxicillin alone potentially inadequate for these pathogens 2

Clinical Efficacy Data

• A 2021 randomized trial found that high-dose amoxicillin/clavulanate (875 mg amoxicillin twice daily plus additional 875 mg) provided no additional benefit over standard-dose (875 mg twice daily) for acute sinusitis, with 44.3% vs 36.4% reporting significant improvement at day 3 (not statistically significant) 5
• This suggests that for uncomplicated sinusitis, standard dosing is appropriate and higher doses do not improve outcomes 5

Adverse Effects

• Diarrhea is common, occurring in approximately 40% of patients by day 3 of treatment, with severe diarrhea in about 7% 5
• The 875 mg twice-daily regimen may cause less moderate-to-severe diarrhea compared to 500 mg three times daily 3