Polymyxin B sulfate and trimethoprim is a prescription antibiotic eye drop used to treat bacterial eye infections, most commonly conjunctivitis (pink eye) and blepharoconjunctivitis. It combines two antibiotics that work in different ways, giving it broad coverage against both gram-positive and gram-negative bacteria. The solution is typically prescribed as one drop every three hours for 7 to 10 days.
What This Combination Treats
This medication is specifically designed for bacterial infections on the surface of the eye. Its two main uses are conjunctivitis, an infection of the thin membrane covering the white of the eye and the inner eyelid, and blepharoconjunctivitis, which involves the same membrane plus the outer eyelid. Both conditions commonly cause redness, discharge, crusting, and a gritty feeling in the eye.
The two most frequently isolated bacteria in acute conjunctivitis are Haemophilus influenzae and Streptococcus pneumoniae. Because one is gram-negative and the other gram-positive, effective treatment needs to cover both categories. That’s exactly why these two antibiotics are paired together: polymyxin B handles gram-negative bacteria well, while trimethoprim adds gram-positive coverage, creating a broad-spectrum combination in a single drop.
This medication does not work against viral or fungal eye infections. Since viruses are actually a common cause of conjunctivitis, not every case of pink eye calls for antibiotic drops. Your prescriber will determine whether a bacterial infection is likely based on your symptoms and the appearance of your eye.
How the Two Antibiotics Work Together
Polymyxin B kills bacteria by physically disrupting their outer membranes. The molecule carries multiple positive electrical charges that bind to negatively charged components on the bacterial surface. Once attached, polymyxin B destabilizes the membrane structure, essentially poking holes in the bacteria’s protective wall. Water rushes in, the membrane breaks down, and the cell dies. This mechanism is particularly effective against gram-negative bacteria, which have a distinctive outer membrane layer that polymyxin B targets.
Trimethoprim takes a completely different approach. It blocks a specific step bacteria need to produce folic acid, a vitamin essential for making DNA. Without active folic acid, bacteria can’t replicate. Human cells get folic acid from food rather than manufacturing it through this same pathway, which is why trimethoprim can target bacteria without harming your own tissue.
Because these two antibiotics attack bacteria through unrelated mechanisms, one destroying the outer membrane and the other shutting down DNA production, they’re effective against a wider range of species than either would be alone. This also makes it harder for bacteria to develop resistance, since surviving one attack mechanism doesn’t protect against the other.
How to Use the Eye Drops
The standard dosing is one drop in the affected eye every three hours, with a maximum of six doses per day. Treatment typically lasts 7 to 10 days. During severe infections, your prescriber may increase the frequency early on. The drops are approved for adults and children 2 months of age and older.
Finishing the full course matters even if your eye looks and feels better after a few days. Stopping early can leave enough bacteria alive to regrow and potentially come back stronger. If symptoms haven’t improved after several days of use, or if they get worse, that’s worth a follow-up visit since a different type of infection or a resistant strain could be involved.
Common Side Effects
Because the drops are applied directly to the eye rather than taken by mouth, the medication stays mostly at the surface and doesn’t circulate through your body in significant amounts. This limits side effects primarily to the eye itself. The most commonly reported reactions are mild burning or stinging when the drop goes in, temporary redness, and itching. These effects are typically brief and settle within minutes.
An allergic reaction is possible, particularly if you have a known sensitivity to either polymyxin B or trimethoprim. Signs of an allergic response include increasing redness, swelling, or itching that gets worse rather than better with continued use.
Storage and Handling
Store the solution at room temperature, between 68°F and 77°F (20°C to 25°C). Keep the bottle protected from light, so storing it in the original box or a medicine cabinet rather than leaving it on a countertop is a good idea. Avoid touching the dropper tip to your eye, fingers, or any other surface, since contaminating the tip can introduce new bacteria into the solution and potentially reinfect your eye.