What is the appropriate dosing regimen for 0.1% polymyxin B sulfate/0.1% trimethoprim ophthalmic drops (Polytrim) in uncomplicated bacterial conjunctivitis for patients older than 12 months (including contact‑lens wearers), and how should treatment be modified for infants younger than 12 months or when MRSA is suspected or the medication is unavailable?

Polymyxin B Sulfate/Trimethoprim Ophthalmic Solution Dosing for Bacterial Conjunctivitis

For uncomplicated bacterial conjunctivitis in patients older than 2 months (including contact lens wearers), instill 1 drop of polymyxin B sulfate 0.1%/trimethoprim 0.1% (Polytrim) in the affected eye(s) every 3 hours while awake (maximum 6 doses per day) for 7 to 10 days. 1

Standard Dosing Regimen

• Adults and children >2 months: Administer 1 drop every 3 hours (maximum 6 doses daily) for 7-10 days in mild to moderate bacterial conjunctivitis 1
• The FDA-approved dosing is identical for pediatric patients over 2 months and adults, with clinical studies demonstrating safety and efficacy in this age group 1
• Most patients should show reduced discharge, pain, and lid edema within 24-48 hours; if no improvement by 48 hours, consider switching to a fluoroquinolone or obtaining cultures 2

Critical Limitations and When NOT to Use Polytrim

Polytrim is insufficient as monotherapy for several important clinical scenarios:

• Contact lens-related infections: Fluoroquinolones (ofloxacin, ciprofloxacin, moxifloxacin) are strongly preferred over Polytrim because contact lens wear increases risk of Pseudomonas aeruginosa infection, which requires broader coverage 3, 2
• Gonococcal conjunctivitis: Requires systemic ceftriaxone 250 mg IM single dose (adults) or 25-50 mg/kg IV/IM (neonates, max 250 mg) plus azithromycin 1 g orally; topical Polytrim alone is inadequate 3, 2
• Chlamydial conjunctivitis: Requires systemic azithromycin 1 g orally single dose OR doxycycline 100 mg orally twice daily for 7 days; topical therapy alone is insufficient 3, 2
• MRSA conjunctivitis: May require compounded topical vancomycin, as MRSA organisms are resistant to polymyxin B and trimethoprim 3
• Severe bacterial keratitis: Requires topical fluoroquinolones with intensive loading regimens (every 5-15 minutes initially, then hourly) due to superior corneal penetration 2

Special Population: Infants <12 Months

• Infants younger than 2 months: Polytrim safety and efficacy have not been established in this age group per FDA labeling 1
• Neonatal conjunctivitis: Requires immediate ophthalmology consultation and systemic antibiotics, as gonococcal or chlamydial infection can cause severe complications including corneal perforation 3, 2
• For neonatal chlamydial conjunctivitis specifically, oral erythromycin 50 mg/kg/day divided into 4 doses for 14 days is recommended 3

Contact Lens Management

• Lens removal is mandatory: Patients must remove contact lenses before instilling drops and should not re-insert lenses until infection has completely resolved 2
• Discard lens case: After resolution, the contact lens case must be replaced with a new sterile case 2
• Consider fluoroquinolone instead: Because Polytrim has limited Pseudomonas coverage, fluoroquinolones are the preferred first-line agents for contact lens wearers 3, 2

When Polytrim is Unavailable or Contraindicated

Alternative topical antibiotics for uncomplicated bacterial conjunctivitis include:

• Fluoroquinolones: Moxifloxacin or gatifloxacin (fourth-generation with superior gram-positive coverage including some MRSA strains) or ofloxacin/ciprofloxacin (third-generation) 3, 2
• Aminoglycosides: Tobramycin 0.3% or gentamicin (WHO-endorsed alternatives) 3, 2
• Macrolides: Azithromycin 1% ophthalmic solution 2
• Tetracyclines: Tetracycline ointment (WHO-endorsed) 3

Comparative Efficacy Evidence

• A pediatric multicenter trial found moxifloxacin 0.5% three times daily achieved complete resolution in 81% of patients by 48 hours versus only 44% with Polytrim four times daily (P=0.001), demonstrating significantly faster clinical cure 4
• However, older studies showed Polytrim eliminated bacteria with equal effectiveness to neomycin-polymyxin B-gramicidin (Neosporin) in conjunctivitis and blepharitis 5
• The American Academy of Ophthalmology states there is no clinical evidence suggesting superiority of any particular antibiotic for mild cases in immunocompetent patients without contact lens use, so the most convenient or least expensive option can be selected 3

Red Flags Requiring Immediate Ophthalmology Referral

Refer urgently if any of the following are present: 3, 2

• Visual loss or decreased vision
• Moderate to severe pain (beyond mild irritation)
• Severe purulent discharge with marked inflammation
• Corneal involvement (infiltrate, ulcer, or opacity)
• Conjunctival scarring or membrane formation
• Lack of response after 3-4 days of appropriate therapy
• Recurrent episodes
• Immunocompromised state or history of HSV eye disease

Important Clinical Pitfalls

• Do not taper below 3-4 times daily before completing the full course, as subtherapeutic dosing increases antibiotic resistance risk 2
• Avoid prolonged use beyond 10 days without ophthalmology consultation, as medication toxicity can cause corneal epithelial damage and worsening inflammation 2
• Do not use for viral conjunctivitis: Antibiotics provide no benefit and promote unnecessary resistance; viral conjunctivitis is self-limited and requires only supportive care 3
• Counsel on infection control: Emphasize frequent hand washing with soap and water, avoiding eye rubbing, using separate towels, and avoiding close contact during the contagious period 3