Natacyn Side Effects: What's Normal and What's Not

If your doctor has prescribed Natacyn (Natamycin 5%) for a fungal eye infection, you're probably wondering what side effects to expect. Like all medications, Natacyn can cause side effects — but the good news is that most are mild and temporary. Knowing the difference between a normal reaction and a warning sign can help you feel more confident during treatment and know when to pick up the phone.

What Is Natacyn?

Natacyn is the only FDA-approved ophthalmic antifungal suspension available in the United States. It contains Natamycin 5%, a polyene antifungal that works by binding to fungal cell membranes and destroying the infection. It's primarily used to treat fungal keratitis (a corneal infection), fungal conjunctivitis, and fungal blepharitis. For a complete overview, see our guide on what Natacyn is and how it's used.

Common Side Effects

These side effects are reported frequently and are usually mild and temporary. They often improve as your eyes adjust to the medication:

• Mild eye irritation or discomfort — A general feeling of irritation in the treated eye is the most commonly reported side effect. This typically fades within a few minutes of applying the drops.
• Stinging or burning — You may feel a brief stinging or burning sensation immediately after instilling the drops. This is normal and usually lasts less than a minute.
• Eye redness (hyperemia) — Some additional redness in the treated eye is common. Keep in mind that your eye is already inflamed from the infection, so it can be hard to tell what's from the medication versus the infection itself.
• Tearing — Your eye may water more than usual after applying Natacyn. This is a normal reflex response.
• Foreign body sensation — It may feel like something is in your eye. Natacyn is a suspension (not a clear solution), so the tiny particles can temporarily create this feeling.
• Blurred vision — Because Natacyn is a milky-white suspension, it can temporarily blur your vision after application. This usually clears within 15-30 minutes. Avoid driving or operating machinery until your vision returns to normal.

These side effects are generally not a reason to stop treatment. Fungal keratitis is a serious, sight-threatening condition, and the benefits of continuing Natacyn far outweigh the discomfort of mild side effects.

Serious Side Effects

While uncommon, some side effects require immediate medical attention. Contact your doctor or go to the emergency room if you experience:

• Allergic reaction — Signs include hives, difficulty breathing, or swelling of your face, lips, tongue, or throat. This is rare but requires emergency care.
• Worsening vision changes — Some blurring right after drops is normal, but if your vision is getting progressively worse over days, tell your doctor. This could signal a complication from the infection or a reaction to the medication.
• Corneal opacity — If you notice a white or cloudy area on your eye that seems to be growing, report it to your doctor. While this can be part of the infection itself, it may also indicate a complication.
• Significant eye swelling (edema) — Mild puffiness is one thing, but pronounced swelling of the eyelid or the eye itself should be evaluated.
• Severe eye pain — Pain that worsens despite treatment or is significantly more intense than when you started Natacyn needs medical evaluation.
• Chest pain or shortness of breath — Though extremely rare with an eye drop, any systemic symptoms like chest pain or difficulty breathing after using Natacyn should be treated as a medical emergency.

Side Effects in Specific Populations

Pregnant or Breastfeeding Women

Natacyn is classified as Pregnancy Category C. No animal reproduction studies have been conducted with Natamycin. It should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus. It's also not known whether Natamycin is excreted in breast milk, so caution is advised for nursing mothers. Talk to your doctor about the risks and benefits.

Children

Safety and effectiveness of Natacyn in pediatric patients have not been established by the manufacturer. However, doctors may prescribe it off-label for children with fungal eye infections when no better alternative exists. Your child's ophthalmologist will weigh the risks and benefits.

Older Adults

No differences in safety or effectiveness have been observed between elderly and younger patients. No dose adjustment is needed for older adults.

How to Manage Common Side Effects

Here are some practical tips to minimize discomfort while using Natacyn:

• Shake the bottle well before each use. Natacyn is a suspension, and proper shaking ensures even distribution of the medication and may reduce irritation from concentrated particles.
• Close your eyes gently after instilling the drop and press lightly on the inner corner of your eye (near your nose) for 1-2 minutes. This reduces drainage and helps the medication stay in your eye.
• Wait before using other eye drops — If you've been prescribed additional eye medications, wait at least 5 minutes between drops to avoid washing out the Natacyn.
• Don't rub your eyes — Even if they feel irritated, rubbing can worsen inflammation and potentially spread the infection.
• Keep a side effect log — Note the time, which side effects you experience, and how long they last. This information is valuable for your follow-up appointments.
• Use artificial tears if approved by your doctor — Some doctors allow preservative-free artificial tears between Natacyn doses to help with dryness and irritation. Always ask first.

When to Call Your Doctor

As a general rule, contact your ophthalmologist if:

1. Your symptoms are getting worse instead of better after 3-4 days of treatment
2. You develop new symptoms that weren't present before starting Natacyn
3. You experience any of the serious side effects listed above
4. You're having trouble keeping up with the dosing schedule (every 1-2 hours initially) due to side effects

Don't stop Natacyn on your own because of side effects. Stopping treatment prematurely can allow the fungal infection to worsen or return. If side effects are making it difficult to continue, your doctor may adjust the dosing schedule or add supportive treatments.

Final Thoughts

Most Natacyn side effects are mild — stinging, tearing, blurred vision — and resolve quickly. Serious reactions are rare. The most important thing is to complete your full course of treatment, which typically lasts 14-21 days, even if your eye starts feeling better before then.

If you're having trouble finding Natacyn to fill your prescription, Medfinder can help you locate pharmacies that have it in stock. And if cost is a concern, check out our guide to saving money on Natacyn.