Polymyxin B-Trimethoprim Dosing for Bacterial Conjunctivitis
For mild to moderate bacterial conjunctivitis, instill one drop of polymyxin B-trimethoprim ophthalmic solution in the affected eye(s) every 3 hours (maximum 6 doses per day) for 7-10 days. 1
Standard Dosing Regimen
- Adults: Apply 1 drop every 3 hours while awake, up to 6 times daily for 7-10 days 1
- Pediatric patients (>2 months): Use the same dosing regimen as adults—1 drop every 3 hours (maximum 6 doses/day) for 7-10 days 1
Clinical Context and Treatment Selection
While polymyxin B-trimethoprim is FDA-approved and effective for bacterial conjunctivitis, newer fluoroquinolones like moxifloxacin demonstrate superior speed of clinical efficacy. 2 In a head-to-head comparison, moxifloxacin achieved complete resolution in 81% of patients by 48 hours versus only 44% with polymyxin-trimethoprim (P=0.001). 2
When to Use Polymyxin B-Trimethoprim:
- Mild to moderate bacterial conjunctivitis with purulent discharge, pain, and marked inflammation 3
- When fluoroquinolones are unavailable or cost is a limiting factor 4
- Proven efficacy against common pathogens including Haemophilus influenzae and Streptococcus pneumoniae 5, 6
Treatment Efficacy Timeline:
- By days 3-6: Expect 47% cure rate and 45% improvement with polymyxin-trimethoprim 5
- By days 8-10: Expect 84-91% cure rate 7, 5
- Bacterial eradication occurs in 71% by day 3-5 and 79% by day 8-10 7
Follow-up and Reassessment
- Return for evaluation if no improvement after 3-4 days of treatment 3
- For severe cases, reassess within 48-72 hours to confirm treatment response 3
- Signs of improvement include: reduced pain, decreased discharge, lessened eyelid edema, and reduced conjunctival injection 3
- If no improvement after 48 hours, consider culture and sensitivity testing and potential therapy change 3, 8
Important Caveats
- Do not continue beyond recommended duration without reassessment, as prolonged topical antibiotic use can cause toxicity 3
- For suspected gonococcal or chlamydial conjunctivitis, systemic antibiotics are required in addition to topical therapy 8
- In neonates with ophthalmia neonatorum, systemic antibiotics may be necessary depending on the causative organism 3
- Mild bacterial conjunctivitis may resolve spontaneously in immunocompetent adults, but treatment reduces transmissibility and allows earlier return to activities 3