What Is Polymyxin B/Trimethoprim? Uses, Dosage, and What You Need to Know in 2026

If your doctor has prescribed Polymyxin B/Trimethoprim eye drops for an eye infection, you probably have a few questions: What is this medication? What does it do? How do I use it? This guide answers all of that in plain English, so you can use it correctly and know what to expect.

What Is Polymyxin B/Trimethoprim?

Polymyxin B/Trimethoprim ophthalmic solution is a prescription antibiotic eye drop used to treat bacterial eye infections. The brand name is Polytrim. It combines two different antibiotics into a single sterile solution:

Polymyxin B sulfate (10,000 units/mL): A polypeptide antibiotic that kills gram-negative bacteria by disrupting their cell membranes.

Trimethoprim sulfate (1 mg/mL): An antifolate antibiotic that kills gram-positive bacteria by blocking their ability to make folic acid — a nutrient bacteria need to reproduce.

Together, these two antibiotics cover a wide range of bacteria that commonly cause eye infections. The FDA approved Polymyxin B/Trimethoprim in October 1988, giving it decades of real-world safety and effectiveness data.

What Is Polymyxin B/Trimethoprim Used For?

Polymyxin B/Trimethoprim is FDA-approved for two conditions:

Acute bacterial conjunctivitis (pink eye): Infection of the membrane that covers the white of the eye and the inside of the eyelid. Bacterial pink eye causes redness, yellow-green discharge, and crusting — especially in the morning.

Blepharoconjunctivitis: A combined infection of the conjunctiva and the eyelid margin, often associated with Staphylococcus bacteria.

It's approved for adults and children 2 months of age and older. It is NOT effective against viral conjunctivitis (caused by viruses like adenovirus or herpes) — antibiotics only work on bacteria.

How Do You Use Polymyxin B/Trimethoprim Eye Drops?

Here's a step-by-step guide to using your eye drops correctly:

Wash your hands thoroughly with soap and water.

Tilt your head back slightly and look up.

Gently pull down your lower eyelid with one finger to create a small pocket.

Hold the dropper tip above the pocket — close, but not touching — and squeeze one drop in.

Gently close your eye and apply light pressure to the inner corner (tear duct) for 1–2 minutes. This keeps the medication in contact with the eye and limits absorption through the tear duct.

Do not touch the dropper tip to any surface, including your eye or fingers.

Wash your hands again after use.

What Is the Correct Dosage?

The standard dose for both adults and children (≥2 months) is:

1 drop in the affected eye(s) every 3 hours

Maximum: 6 doses per day

Duration: 7–10 days

Complete the full course even if your eye looks better sooner. Stopping antibiotic treatment early can cause the infection to return and can contribute to antibiotic resistance.

How Much Does Polymyxin B/Trimethoprim Cost?

Polymyxin B/Trimethoprim is one of the more affordable antibiotic eye drops available. A 10 mL bottle (one full treatment course) typically costs $12–$32 at retail. With a GoodRx coupon, the price can drop to as low as $5–$10 at many pharmacies. Insurance typically covers it as a Tier 1 or Tier 2 generic with a low or zero copay.

What Should You Not Do While Using These Eye Drops?

Do not wear contact lenses during treatment — the preservative (benzalkonium chloride) can damage soft lenses.

Do not share the bottle with anyone else — this can spread infection.

Do not inject into the eye — this is for surface use only.

Do not use in children under 2 months — safety has not been established for this age group.

The Bottom Line

Polymyxin B/Trimethoprim (Polytrim) is a well-established, affordable antibiotic eye drop for bacterial conjunctivitis and blepharoconjunctivitis. Used correctly over 7–10 days, it clears most bacterial eye infections effectively. To learn more about how the drug kills bacteria, see our article on how Polymyxin B/Trimethoprim works.