Dosing Regimen for Polymyxin B/Trimethoprim Eye Drops for Conjunctivitis
For mild to moderate bacterial conjunctivitis, instill one drop of polymyxin B/trimethoprim in the affected eye(s) every 3 hours (maximum 6 doses per day) for 7-10 days. 1
Standard Dosing Protocol
The FDA-approved regimen for polymyxin B/trimethoprim ophthalmic solution is straightforward and applies to both adults and children over 2 months of age 1:
- Frequency: 1 drop every 3 hours while awake
- Maximum: 6 doses per 24-hour period
- Duration: 7-10 days
- Pediatric dosing: Same as adults for children >2 months 1
Disease Severity Considerations
While the question asks about "Poly-Mix" (which appears to be polymyxin B/trimethoprim based on context), note that if you're referring to a polymyxin B-containing steroid combination, the dosing differs 2:
- Severe disease: Hourly initially, then taper as inflammation subsides
- Mild disease: 4-6 times daily
- Maximum initial prescription: 20 mL without re-evaluation
Clinical Context and Effectiveness
The 2024 AAO guidelines indicate that mild bacterial conjunctivitis is self-limited and a 5-7 day course of broad-spectrum topical antibiotics is usually effective, with no particular antibiotic showing superiority for uncomplicated cases 3. However, polymyxin B/trimethoprim remains clinically effective despite being slower-acting than newer fluoroquinolones 4.
Key Evidence Points:
- Clinical cure rates: Polymyxin B/trimethoprim achieved 96% clinical cure by days 7-10, statistically non-inferior to moxifloxacin 4
- Speed of action: Moxifloxacin shows faster symptom resolution (81% cured by 48 hours vs 44% with polymyxin B/trimethoprim) 5, but final outcomes are equivalent
- Cost-effectiveness: Polymyxin B/trimethoprim offers significant cost savings compared to fluoroquinolones with comparable final cure rates 4
Important Caveats
- Do not prescribe plain trimethoprim eye drops alone - the combination with polymyxin B is essential for adequate gram-negative coverage, particularly Pseudomonas aeruginosa 1
- Moderate to severe cases: Consider fluoroquinolones or obtain cultures if copious purulent discharge, severe pain, or marked inflammation present 3
- MRSA concerns: Polymyxin B/trimethoprim may have limited activity against methicillin-resistant Staphylococcus aureus, which is increasingly common 3
- Limit initial prescription: No more than 20 mL should be prescribed initially, requiring re-evaluation before refills 2
When to Choose Alternatives
Consider newer fluoroquinolones (moxifloxacin) when:
- Faster symptom resolution is critical (school/work return)
- Reducing contagion period is priority
- Severe or rapidly progressive infection
- Known or suspected resistant organisms