Polymyxin B-Trimethoprim Ophthalmic Solution Dosing for Conjunctivitis
For mild to moderate bacterial conjunctivitis, instill 1 drop in the affected eye(s) every 3 hours (maximum 6 doses per day) for 7 to 10 days. 1
Dosing Regimen
- Standard dosing: 1 drop every 3 hours while awake, up to 6 times daily 1
- Treatment duration: 7 to 10 days 1
- Pediatric patients: Same dosing as adults for children over 2 months of age 1
Clinical Response Timeline
- Reassess at 3-4 days: Patients should return if no improvement is noted 2, 3
- Expected response: Look for reduced pain, decreased discharge, lessened lid edema, and reduced conjunctival injection 2
- 48-hour benchmark: Studies show only 44% of patients treated with polymyxin B-trimethoprim achieve complete resolution by 48 hours, compared to 81% with moxifloxacin 4
Important Clinical Considerations
Efficacy limitations: While polymyxin B-trimethoprim is FDA-approved and effective, it demonstrates slower clinical response compared to fourth-generation fluoroquinolones 4. Research shows that by 48 hours, less than half of patients achieve complete symptom resolution with this combination 4.
When to consider alternatives: 2, 5
- If no improvement after 3-4 days of therapy
- For severe bacterial conjunctivitis with marked purulent discharge
- When faster resolution is clinically important to reduce transmission
Spectrum coverage: Polymyxin B-trimethoprim provides broad-spectrum coverage against common conjunctival pathogens including H. influenzae and S. pneumoniae 6, 7, making it appropriate for empiric therapy in mild to moderate cases 3.
Critical Pitfalls to Avoid
- Do not reduce dosing frequency below 3-4 times daily: Lower doses are subtherapeutic and promote resistance 2
- Do not continue beyond recommended duration without reassessment: Prolonged use causes ocular surface toxicity 2, 3
- Do not use as monotherapy for gonococcal or chlamydial conjunctivitis: These require systemic antibiotics 8, 5
Special Populations Requiring Different Management
Neonates with ophthalmia neonatorum: May require systemic antibiotics in addition to topical therapy depending on causative organism 3
Severe bacterial conjunctivitis: Consider fourth-generation fluoroquinolones (moxifloxacin 0.5% or gatifloxacin 0.5%) dosed 3 times daily, which provide superior gram-positive coverage and faster clinical response 2, 5