What is the recommended dose of amoxicillin for an average adult patient with no significant medical history and acute bacterial sinusitis?

Amoxicillin Dosing for Acute Bacterial Sinusitis in Adults

For an average adult with acute bacterial sinusitis and no significant medical history, prescribe amoxicillin-clavulanate 500 mg/125 mg three times daily or 875 mg/125 mg twice daily for 5-7 days. 1

First-Line Treatment Approach

The American College of Physicians and Infectious Diseases Society of America recommend amoxicillin-clavulanate as the preferred first-line treatment over amoxicillin alone. 1 This is critical because approximately 50% of H. influenzae and 90-100% of M. catarrhalis strains produce β-lactamase, rendering plain amoxicillin ineffective. 1

Standard dosing options include:

• 500 mg/125 mg three times daily (every 8 hours) for 5-7 days 1, 2
• 875 mg/125 mg twice daily (every 12 hours) for 5-7 days 1, 2

Both regimens are equally effective, so choose based on patient preference for dosing frequency. 1 All doses should be taken at the start of meals to minimize gastrointestinal side effects. 1

Treatment Duration

The recommended duration is 5-7 days for uncomplicated acute bacterial sinusitis in adults. 1, 2 This represents a shift from older guidelines that recommended 10-14 days. 3, 4 However, some guidelines still suggest continuing treatment until the patient has been symptom-free for 7 days, which typically results in a 10-14 day course. 3, 4

The most recent high-quality evidence supports the shorter 5-7 day duration as equally effective with fewer adverse events. 1

When Plain Amoxicillin May Be Considered

If amoxicillin-clavulanate is not available or tolerated, plain amoxicillin 500 mg three times daily can be used as an alternative, though it is less preferred. 3, 4 This is only reasonable in patients without recent antibiotic use and in areas with low resistance rates. 1

Reassessment Strategy

Evaluate treatment response after 3-5 days. 3, 1, 4 If symptoms worsen or fail to improve:

• Switch to a respiratory fluoroquinolone (levofloxacin or moxifloxacin) 3
• Consider high-dose amoxicillin-clavulanate (2000 mg/125 mg twice daily) if available, though recent evidence questions its superiority 5
• Alternative options include cephalosporins (cefuroxime, cefpodoxime) or macrolides for penicillin-allergic patients 3, 1

Critical Diagnostic Considerations Before Prescribing

Only prescribe antibiotics if the patient meets criteria for bacterial sinusitis: 1, 4

• Persistent symptoms ≥10 days without improvement, OR
• Severe symptoms (fever ≥39°C, purulent nasal discharge, facial pain) for 3-4 consecutive days, OR
• "Double sickening" - worsening after initial improvement

Antibiotics provide no benefit for post-viral rhinosinusitis and increase adverse events. 4, 2 This is a common pitfall that contributes to antibiotic resistance. 4

High-Dose Considerations

High-dose amoxicillin-clavulanate (2000 mg/125 mg twice daily or 90 mg/kg/day) should be considered if: 1

• Recent antibiotic use within the past 30 days
• Moderate to severe disease
• Living in an area with >10% prevalence of penicillin-resistant S. pneumoniae
• Daycare attendance or similar high-exposure settings

However, a 2021 randomized trial found no benefit of high-dose over standard-dose amoxicillin-clavulanate, with similar efficacy but potentially more gastrointestinal side effects. 5 Therefore, standard dosing is preferred for most patients. 1

Adjunctive Therapies

Recommend intranasal saline irrigation and intranasal corticosteroids as adjunctive treatments to improve symptom relief. 1 These may be more effective than antibiotics alone for symptom management. 4

Avoid topical or oral decongestants and antihistamines as they provide no proven benefit. 1

Common Pitfalls to Avoid

• Do not prescribe antibiotics for viral upper respiratory infections - studies show no benefit and increased adverse events 4, 2
• Do not use first-generation cephalosporins (cephalexin, cefadroxil) as they have poor coverage against H. influenzae 1
• Do not use cefixime or ceftibuten as they have inadequate activity against S. pneumoniae 1
• Do not routinely cover for MRSA during initial empiric therapy 1
• Do not stop antibiotics prematurely when symptoms improve, as this leads to incomplete eradication and potential relapse 4, 2

Expected Adverse Effects

The most common adverse effects are gastrointestinal: diarrhea (most common), nausea, vomiting, and vaginitis. 6, 7 Severe diarrhea occurs in approximately 5-8% of patients. 8, 5 Warn patients about these effects and advise taking medication with food. 1