Amoxicillin for Acute Bacterial Sinusitis
For adults with acute bacterial sinusitis, prescribe amoxicillin 500 mg every 8 hours or 875 mg every 12 hours for 5-7 days as first-line therapy, reserving amoxicillin-clavulanate for treatment failure or high-risk patients. 1, 2, 3
Initial Diagnosis and Treatment Decision
Before prescribing any antibiotic, confirm you're dealing with acute bacterial rhinosinusitis (ABRS) rather than viral rhinosinusitis. ABRS is diagnosed when symptoms persist ≥10 days without improvement, or when there's "double worsening" (initial improvement followed by worsening within 10 days), or severe symptoms (high fever ≥39°C plus purulent nasal discharge for ≥3-4 consecutive days) 2, 3.
Watchful waiting without antibiotics is appropriate for all patients with uncomplicated ABRS, regardless of severity, provided you can ensure follow-up within 7 days 1, 3. This represents a significant shift from older guidelines and reflects evidence that many patients improve spontaneously. The 2025 AAO-HNS guideline explicitly extends this option to all severity levels, not just mild cases 3.
First-Line Antibiotic Selection
Standard Amoxicillin vs. Amoxicillin-Clavulanate
The evidence strongly supports amoxicillin alone as first-line therapy for most adults:
A 2026 retrospective cohort study of 234,608 adults found no difference in treatment failure between standard-dose amoxicillin-clavulanate and amoxicillin (3.0% vs 3.1%), but amoxicillin-clavulanate caused significantly more yeast infections (1.1% vs 0.8%) and C. difficile infections (0.04% vs 0.02%) 4. This is the highest quality, most recent evidence available.
The 2015 AAO-HNS guideline recommends amoxicillin with or without clavulanate, emphasizing cost-effectiveness and reduced adverse events with amoxicillin alone 1.
The 2012 IDSA guideline recommends amoxicillin-clavulanate for both children and adults, but this predates the 2026 comparative effectiveness data 2.
The 2025 AAO-HNS guideline now explicitly recommends amoxicillin with or without clavulanate, reflecting the evolving evidence base 3.
Dosing Regimen
Adults:
- 500 mg every 8 hours, OR
- 875 mg every 12 hours 5
Children ≥3 months and <40 kg:
- Mild/moderate: 25 mg/kg/day divided every 12 hours or 20 mg/kg/day divided every 8 hours
- Severe: 45 mg/kg/day divided every 12 hours or 40 mg/kg/day divided every 8 hours 5
Duration of Therapy
5-7 days is adequate for adults with uncomplicated ABRS 1, 2, 3. This shorter duration reduces adverse events without compromising efficacy. The traditional 10-14 day courses are no longer necessary for most patients.
Children should receive 10-14 days of treatment 2.
When to Use Amoxicillin-Clavulanate
Reserve amoxicillin-clavulanate for:
- Treatment failure after 7 days of amoxicillin or watchful waiting 1, 3
- High-risk patients with recent antibiotic use (within 30 days), known local resistance patterns, or immunocompromising conditions 2
- Severe disease requiring broader coverage 6
When prescribing amoxicillin-clavulanate, use high-dose formulation: 2000 mg (amoxicillin component) every 12 hours or 90 mg/kg/day in children to overcome resistant Streptococcus pneumoniae 6, 2.
Critical Pitfall: Post-Viral Rhinosinusitis
Do not prescribe antibiotics for post-viral acute rhinosinusitis (symptoms <10 days following a viral upper respiratory infection). The 2020 EPOS guideline provides moderate-quality evidence that antibiotics offer no benefit in this population and cause more adverse events 7. This is a common prescribing error that contributes to antibiotic resistance.
Treatment Failure Management
If the patient fails to improve after 7 days or worsens at any time:
- Reassess the diagnosis - exclude complications, other diagnoses, or chronic rhinosinusitis 1, 3
- If initially managed with watchful waiting: Start amoxicillin 1
- If initially treated with amoxicillin: Switch to high-dose amoxicillin-clavulanate 1, 2
- Consider alternative agents for penicillin allergy: respiratory fluoroquinolones (levofloxacin, moxifloxacin) or doxycycline 1, 2
High-Dose Controversy
Two recent RCTs examined whether high-dose amoxicillin (1750-2000 mg twice daily) improves outcomes:
- A 2021 trial found no benefit of high-dose over standard-dose amoxicillin-clavulanate 8
- A 2018 trial showed immediate-release high-dose improved symptoms at day 3 but caused more severe diarrhea (15.8% vs 4.8%), while extended-release high-dose showed no benefit 9
Standard dosing remains appropriate for initial therapy unless treating documented resistant organisms 1, 4.
Adjunctive Therapies
- Intranasal corticosteroids are recommended as adjuncts, particularly in patients with allergic rhinitis 2, 3
- Saline irrigation provides modest symptomatic benefit 2
- Avoid antihistamines and decongestants - no proven benefit in ABRS 6
Special Populations
Renal impairment (GFR <30 mL/min): Reduce to 500 mg or 250 mg every 12-24 hours depending on severity 5
Pregnancy: Amoxicillin is generally safe, but document clear indication given the modest benefits of antibiotic therapy 1