Is doxycycline appropriate for acute bacterial sinusitis in an adult (e.g., with penicillin allergy), and what is the recommended dosing regimen?

Doxycycline for Sinus Infection

Doxycycline is an acceptable but suboptimal alternative for acute bacterial sinusitis, particularly in penicillin-allergic patients, but it should not be your first choice due to its significantly higher failure rate (20–25%) compared to amoxicillin-clavulanate (8–10%). 1

When Doxycycline Is Appropriate

Doxycycline 100 mg once daily for 10 days is reserved for specific scenarios where better alternatives cannot be used: 1

• Documented penicillin allergy where second- or third-generation cephalosporins are also contraindicated or refused 1
• Mild disease in patients who have not received antibiotics in the previous 4–6 weeks 1
• When fluoroquinolones must be avoided (e.g., pregnancy, tendon disorders, QT-prolongation risk, or in children < 18 years) 1

However, before prescribing doxycycline, you should first consider second- or third-generation cephalosporins (cefuroxime, cefpodoxime, cefdinir) in penicillin-allergic patients, as cross-reactivity is negligible and their efficacy (90–92%) far exceeds doxycycline (77–81%). 1

Why Doxycycline Is Not First-Line

The predicted clinical efficacy of doxycycline is only 77–81% with a 20–25% bacteriologic failure rate, compared to amoxicillin-clavulanate's 90–92% efficacy and 8–10% failure rate. 1 This inferior performance stems from:

• Limited activity against Haemophilus influenzae (30–40% of strains produce β-lactamase, rendering doxycycline ineffective) 1
• Inadequate coverage of penicillin-resistant Streptococcus pneumoniae 1

Confirming Bacterial Sinusitis Before Prescribing

Do not prescribe antibiotics—including doxycycline—unless the patient meets at least one of these three diagnostic criteria: 1

1. Persistent symptoms ≥ 10 days without improvement (purulent nasal discharge plus obstruction or facial pain/pressure)
2. Severe symptoms ≥ 3–4 consecutive days (fever ≥ 39°C with purulent discharge and facial pain)
3. "Double sickening" (initial improvement from a viral URI followed by worsening within 10 days)

Remember: 98–99.5% of acute rhinosinusitis is viral and resolves spontaneously within 7–10 days without antibiotics. 1

Dosing Regimen

Standard adult dose: Doxycycline 100 mg orally once daily for 10 days (or until symptom-free for 7 consecutive days, typically 10–14 days total). 1

Doxycycline is contraindicated in children < 8 years due to risk of permanent tooth enamel discoloration. 1

Monitoring and Switching Antibiotics

• Reassess at 3–5 days: If no clinical improvement (persistent purulent drainage, unchanged facial pain, or worsening), immediately switch to amoxicillin-clavulanate 875 mg/125 mg twice daily or a respiratory fluoroquinolone (levofloxacin 500 mg once daily or moxifloxacin 400 mg once daily). 1
• Reassess at 7 days: Persistent or worsening symptoms warrant confirmation of diagnosis, exclusion of complications (orbital cellulitis, meningitis), and consideration of imaging or ENT referral. 1

Essential Adjunctive Therapies (Add to All Patients)

These therapies significantly improve outcomes regardless of antibiotic choice: 1

• Intranasal corticosteroids (mometasone, fluticasone, or budesonide) twice daily—reduce mucosal inflammation and accelerate symptom resolution (strong evidence from multiple RCTs) 1
• Saline nasal irrigation 2–3 times daily—provides symptomatic relief and enhances mucus clearance 1
• Analgesics (acetaminophen or ibuprofen)—for pain and fever control 1

Critical Pitfalls to Avoid

• Do not use doxycycline as first-line therapy in patients without a penicillin allergy—this raises the failure rate from 8–10% to 20–25%. 1
• Counsel patients on photosensitivity—advise sun protection and avoidance of tanning beds. 1
• Instruct patients to take doxycycline with a full glass of water while remaining upright to prevent rare esophageal caustic burns. 1
• Ensure adequate treatment duration (minimum 10 days) to prevent relapse. 1
• Do not prescribe antibiotics for symptoms < 10 days unless severe features (fever ≥ 39°C with purulent discharge for ≥ 3 consecutive days) are present. 1

Referral to ENT

Refer immediately if: 1

• No improvement after 7 days of appropriate second-line antibiotic therapy
• Worsening symptoms at any point (increasing pain, fever, drainage)
• Signs of complications (severe headache, visual changes, periorbital swelling, altered mental status)
• Recurrent sinusitis (≥ 3 episodes per year)