Polymyxin B/Trimethoprim Side Effects: What to Expect and When to Call Your Doctor

Polymyxin B/Trimethoprim (Polytrim) is generally well tolerated, especially because it's applied directly to the eye and very little is absorbed into the bloodstream. But like all medications, it can cause side effects — most of them mild and temporary. This guide covers what to expect during treatment, which side effects warrant a call to your doctor, and how to use the drops safely.

Common Side Effects (Normal and Expected)

The most frequently reported side effects of Polymyxin B/Trimethoprim ophthalmic solution are local eye reactions. These are normal and usually mild:

Stinging or burning: A brief stinging sensation immediately after instilling the drops is common. This typically fades within 30–60 seconds.

Redness: Mild increased redness after instillation. Some initial redness from the infection itself is expected and should improve with treatment.

Itching: Mild itching at the site of application may occur, especially with extended use.

Temporary blurred vision: Drops can temporarily blur vision immediately after use. Wait a few minutes before driving or doing tasks that require sharp vision.

Less Common Side Effects

Lid edema (swelling of the eyelid): Can indicate a hypersensitivity reaction. Monitor for worsening.

Tearing: Increased watery discharge may occur as a hypersensitivity response.

Circumocular rash: A rash around the eye area may indicate an allergic reaction. Stop using the drops and contact your doctor.

Photosensitivity: Sensitivity to light has been reported, primarily associated with oral trimethoprim use; rare with topical application.

Serious Side Effects: When to Stop the Drops and Seek Help

Discontinue use and contact your doctor or go to urgent care immediately if you experience:

Worsening redness, pain, or swelling: If symptoms are getting worse rather than better after 48–72 hours of treatment, this could indicate resistance or a non-bacterial infection.

Signs of a serious allergic reaction: Hives, difficulty breathing, swelling of the face, lips, tongue, or throat require emergency medical attention.

Vision changes: Persistent or worsening blurred vision beyond the brief post-drop period is not a normal side effect and should be evaluated.

Fungal superinfection: With prolonged antibiotic use, fungi can overgrow in the eye. Signs include new discharge, severe irritation, or symptoms that return after initial improvement. Do not extend use beyond the prescribed 7–10 days.

Contact Lens Warning

Do not wear contact lenses while you have symptoms of bacterial conjunctivitis or while using Polymyxin B/Trimethoprim eye drops. The preservative in the drops (benzalkonium chloride 0.004%) can be absorbed by soft contact lenses and discolor or damage them. If your doctor says you can wear contacts during treatment, remove them before instilling drops and wait at least 15 minutes before reinserting.

Tips to Minimize Side Effects

After instilling drops, gently close your eye and apply light pressure to the inner corner of the eye (tear duct) for 1–2 minutes. This reduces systemic absorption and can minimize local irritation.

Don't touch the dropper tip to any surface, including your eye — this can contaminate the bottle and worsen infection.

Store the bottle at room temperature (59°F–77°F) and protect from light. Don't refrigerate.

Complete the full course of treatment (7–10 days), even if symptoms improve after 2–3 days. Stopping early can lead to treatment failure or antibiotic resistance.

Contraindications: Who Should Not Use This Drug

Do not use Polymyxin B/Trimethoprim if you are allergic to either polymyxin B or trimethoprim. Let your doctor know about all allergies before starting treatment. This medication is not approved for infants younger than 2 months of age. Also, this solution is specifically for topical use on the eye — it must never be injected into the eye.

The Bottom Line

Polymyxin B/Trimethoprim eye drops are safe and well tolerated for most people. Mild stinging, temporary redness, and brief blurred vision are expected and normal. If symptoms worsen or you develop signs of an allergic reaction, stop use and contact your provider. For information about drug interactions, see our guide on Polymyxin B/Trimethoprim drug interactions.