What is the recommended dosage and treatment duration for Amoxicillin (amoxicillin) in adults with bacterial infections?

Amoxicillin Dosing for Adults with Bacterial Infections

For most adult bacterial infections, amoxicillin should be dosed at 500 mg every 8 hours or 875 mg every 12 hours for mild-to-moderate infections, and 875 mg every 12 hours or 500 mg every 8 hours for severe infections, with treatment duration of 7-10 days for respiratory infections and at least 10 days for streptococcal infections. 1

Standard Adult Dosing by Infection Severity

Mild-to-Moderate Infections

• Ear/Nose/Throat, Skin/Skin Structure, Genitourinary Tract: 500 mg every 12 hours OR 250 mg every 8 hours 1
• This dosing provides adequate coverage for most community-acquired pathogens including Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis 2

Severe Infections

• All sites including Lower Respiratory Tract: 875 mg every 12 hours OR 500 mg every 8 hours 1
• The 875 mg twice-daily formulation is specifically recommended for respiratory infections including acute bacterial rhinosinusitis, community-acquired pneumonia, and acute exacerbations of chronic bronchitis 3
• Lower respiratory tract infections require the higher dose (875 mg every 12 hours or 500 mg every 8 hours) regardless of severity 1

Specific Clinical Scenarios

Community-Acquired Pneumonia

• For outpatients with cardiopulmonary disease or modifying factors: High-dose amoxicillin (1 g every 8 hours) is recommended as part of combination therapy with a macrolide or doxycycline 4
• This higher dosing targets drug-resistant S. pneumoniae (DRSP) and provides adequate coverage for H. influenzae 4

Acute Bacterial Rhinosinusitis

• Mild disease without recent antibiotic use: 1.5-4 g/day total daily dose, with lower doses (1.5 g/day) appropriate for patients without risk factors 4
• Moderate disease or recent antibiotic use (past 4-6 weeks): 4 g/day (approximately 1.3 g every 8 hours or 2 g every 12 hours) 4
• Higher doses are advantageous in areas with high prevalence of penicillin-resistant S. pneumoniae or for patients with risk factors for resistant pathogens 4

Streptococcal Pharyngitis

• Standard dosing: 1000 mg once daily for 10 days (alternative to penicillin V) 3
• Minimum 10-day treatment duration is mandatory to prevent acute rheumatic fever, regardless of clinical improvement 1, 3

H. pylori Infection

• Triple therapy: 1 gram amoxicillin twice daily (every 12 hours) 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

Treatment Duration Guidelines

Respiratory Infections

• Typical course: 7-10 days 5
• Acute bacterial rhinosinusitis in adults: 5-7 days 5, 3
• Treatment should continue for minimum 48-72 hours beyond symptom resolution or evidence of bacterial eradication 1

Streptococcal Infections

• Mandatory minimum: 10 days for any Streptococcus pyogenes infection to prevent acute rheumatic fever 1, 3
• Some infections may require several weeks of therapy with clinical/bacteriological follow-up for several months after cessation 1

Dosing Adjustments for Renal Impairment

Severe Renal Dysfunction

• 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
• Critical caveat: Patients with GFR <30 mL/min should NOT receive the 875 mg dose 1

Administration Considerations

Timing with Meals

• Take at the start of a meal to minimize gastrointestinal intolerance 1
• This is particularly important given that diarrhea is the most common adverse reaction (>1% incidence) 1

Clinical Response Assessment

• Evaluate within 48-72 hours of initiating therapy 5
• Failure to improve after 72 hours should prompt either switch to alternate antimicrobial therapy or patient reevaluation 4
• For acute bacterial rhinosinusitis, if no improvement occurs with initial therapy, consider switching to respiratory fluoroquinolones (gatifloxacin, levofloxacin, moxifloxacin) or amoxicillin-clavulanate 4

Critical Pitfalls to Avoid

Resistance Considerations

• Recent antibiotic use (past 4-6 weeks) is a major risk factor for infection with resistant organisms and should prompt higher dosing or alternative agents 4
• In areas with high prevalence of penicillin-resistant S. pneumoniae, standard doses may be insufficient 4
• Consider amoxicillin-clavulanate instead of amoxicillin alone when beta-lactamase-producing organisms (H. influenzae, M. catarrhalis) are suspected 4, 2

Contraindications

• Absolute contraindication: History of serious hypersensitivity reaction (anaphylaxis or Stevens-Johnson syndrome) to amoxicillin or other beta-lactams 1
• Serious and occasionally fatal anaphylactic reactions have been reported, particularly in patients with history of penicillin hypersensitivity or multiple allergen sensitivities 1

Inadequate Dosing

• Research suggests that standard doses may not achieve pharmacodynamic targets (40% time above MIC) in all hospitalized patients, particularly for organisms with MIC of 8 mg/L 6
• The 875 mg every 12 hours formulation produces higher serum levels and is more likely to achieve therapeutic targets than lower doses 7, 8