Cephalexin is NOT Recommended for Sinusitis
Cephalexin should not be used to treat sinusitis because first-generation cephalosporins like cephalexin have inadequate coverage against Haemophilus influenzae, one of the three most common bacterial pathogens in acute bacterial sinusitis, making them inappropriate for this indication. 1
Why Cephalexin Fails in Sinusitis
The three most common bacterial pathogens in acute sinusitis are Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis 1. First-generation cephalosporins like cephalexin and cefadroxil have poor coverage for H. influenzae and are therefore explicitly deemed inappropriate for sinusitis treatment 1.
- Nearly 50% of H. influenzae strains are β-lactamase producing in most geographic areas, rendering cephalexin ineffective against a substantial portion of causative organisms 1
- 90-100% of M. catarrhalis are β-lactamase producing, further limiting cephalexin's utility 1
What You Should Use Instead
First-Line Treatment for Adults
Amoxicillin remains the first-line antibiotic for acute bacterial sinusitis in adults 2:
- Standard dosing: 500 mg twice daily for uncomplicated cases 2
- High-dose: 875 mg twice daily for more severe infections or areas with high prevalence of resistant S. pneumoniae 2
- Treatment duration: 10-14 days or until symptom-free for 7 days 2
Alternative First-Line Options (for penicillin allergy or treatment failure)
If amoxicillin is inappropriate, use second- or third-generation cephalosporins 1, 2:
- Cefuroxime: 250-500 mg twice daily 1
- Cefpodoxime: 200-400 mg twice daily 1
- Cefprozil: 250-500 mg twice daily 1
- Cefdinir: 300 mg twice daily or 600 mg once daily 3
These agents provide adequate coverage against both H. influenzae and S. pneumoniae 1, 2.
Second-Line Treatment (for treatment failure)
High-dose amoxicillin-clavulanate (875 mg/125 mg twice daily) should be used when first-line therapy fails after 3-5 days 2. The clavulanate component provides coverage against β-lactamase-producing organisms 2.
Respiratory fluoroquinolones (levofloxacin 500 mg once daily or moxifloxacin 400 mg once daily) should be reserved for treatment failures, complicated sinusitis, or patients with severe penicillin allergy 2, 4.
Historical Context: Why This Matters
While older studies from 1985 and 1994 showed cephalexin had some efficacy in sinusitis 5, 6, these studies predate current understanding of bacterial resistance patterns and were conducted before β-lactamase-producing organisms became highly prevalent 1. The 2005 guidelines from the American Academy of Allergy, Asthma, and Immunology explicitly contraindicate first-generation cephalosporins for sinusitis based on their inadequate H. influenzae coverage 1.
Critical Pitfall to Avoid
Do not prescribe cephalexin for sinusitis simply because it is a "cephalosporin"—generation matters. Only second- and third-generation cephalosporins provide adequate coverage for the polymicrobial nature of acute bacterial sinusitis 1, 2.