Can antibiotic eye drops cause diarrhea?

October 9, 2025 •

4 min read

While about 1 in 5 people taking oral antibiotics experience diarrhea, it is extremely rare for topical antibiotic eye drops to cause this gastrointestinal side effect. The primary reason for this difference lies in the medication's route of administration and the minimal amount absorbed into the body's systemic circulation. This article explores how antibiotic eye drops function and why they are unlikely to disrupt your digestive system.

Quick Summary

Antibiotic eye drops are unlikely to cause diarrhea because they are topically applied with minimal systemic absorption. Unlike oral antibiotics that affect the gut microbiome, eye drops drain primarily through the tear ducts and are absorbed in insufficient amounts to cause gastrointestinal issues. Proper application can further reduce systemic effects.

Key Points

Extremely Rare Side Effect: Diarrhea is an extremely rare side effect of antibiotic eye drops due to minimal systemic absorption, unlike oral antibiotics.
Minimal Systemic Absorption: Most of the eye drop medication is absorbed locally or drained via the nasolacrimal duct, with a very small concentration reaching the bloodstream.
Oral vs. Topical: Oral antibiotics commonly cause diarrhea by disrupting the gut's bacterial balance, which does not happen with the low dose absorbed from eye drops.
Proper Application Matters: Using proper technique, such as punctal occlusion, can further reduce the minor systemic absorption of eye drops and associated side effects.
Localized Side Effects: The most common side effects from antibiotic eye drops are localized to the eye, such as stinging, redness, or blurred vision, not gastrointestinal issues.
Serious Symptoms Need Attention: While unlikely to be caused by eye drops, any severe or persistent gastrointestinal or other systemic symptoms should be reported to a healthcare provider.

Understanding the Routes of Antibiotic Administration

Antibiotics are powerful medications designed to treat bacterial infections. Their side effects, including gastrointestinal issues like diarrhea, are largely dependent on how the medication is delivered to the body. The two most relevant methods for comparison are topical application, such as eye drops, and oral administration, such as pills or liquids. The way your body handles each of these forms dictates the potential for side effects throughout your system.

How Eye Drops are Absorbed into the Body

When you place a drop of medication in your eye, it is not contained solely within the ocular surface. The excess fluid and medication can drain into the nasolacrimal duct system, which connects the corner of your eye to your nose and throat. From there, the medication is swallowed and can be absorbed into the bloodstream via the highly vascular nasal mucosa and gastrointestinal tract.

However, research shows that typically less than 5% of the applied dose of an eye drop actually penetrates the cornea to reach intraocular tissues. A larger fraction is absorbed systemically, but the overall concentration that enters your bloodstream is very low compared to an oral dose. This minimal systemic absorption is the key reason why significant gastrointestinal side effects are not common with eye drops. To further minimize systemic absorption, techniques like punctal occlusion can be used, which involves gently pressing the corner of the eye near the nose for a few minutes after instilling the drops.

Why Oral Antibiotics Can Cause Diarrhea

The most common cause of diarrhea from antibiotics is the disruption of the natural balance of your gut bacteria, or microbiome. Your gastrointestinal tract relies on millions of beneficial bacteria to function properly and protect against harmful pathogens. Oral antibiotics are absorbed into the bloodstream from the gastrointestinal system, reaching bacteria throughout the body, including the gut.

By killing off beneficial bacteria, oral antibiotics allow harmful bacteria to multiply and thrive. A common culprit for severe cases is a bacterium called Clostridioides difficile, or C. diff, which can release toxins that damage the intestinal lining. Because oral antibiotics are designed to have systemic effects, they necessarily impact the gut flora, making gastrointestinal side effects a frequent occurrence.

Can Antibiotic Eye Drops Cause Diarrhea? The Verdict

In short, it is extremely unlikely for antibiotic eye drops to cause diarrhea when used as prescribed. The amount of medication that reaches the digestive system via tear duct drainage is so small that it does not significantly affect the gut microbiome. The risk is vastly different from that associated with oral antibiotics, which are systemically absorbed in therapeutic doses. Concerns about diarrhea stemming from antibiotic eye drops are often based on misinformation or confusion with the side effects of oral medications.

However, no medication is entirely without risk. In very rare circumstances, an individual might have an allergic reaction to the antibiotic that manifests systemically. A rash or other systemic symptoms should be addressed with a healthcare provider. Furthermore, it is crucial to follow all instructions from your doctor and never intentionally ingest eye drops. Over-the-counter eye drops containing vasoconstrictors like tetrahydrozoline are particularly dangerous if swallowed and can cause serious systemic harm.

Comparison: Topical vs. Oral Antibiotics


Feature	Topical (Eye Drops)	Oral (Pills/Liquid)
Absorption Route	Minimal systemic absorption via nasolacrimal duct and conjunctiva.	High systemic absorption via the gastrointestinal tract.
Intended Target	Localized to the eye and surrounding tissues.	Systemic, to reach infections anywhere in the body.
Diarrhea Risk	Extremely rare; negligible when used correctly.	Common; a well-known side effect due to gut microbiome disruption.
Typical Side Effects	Localized irritation, stinging, redness, blurred vision.	Nausea, stomach upset, diarrhea.
Mitigation for Systemic Effects	Punctal occlusion and careful administration.	Taking probiotics and only using when necessary.

Local vs. Systemic Side Effects

Common Local Side Effects: Antibiotic eye drops are primarily associated with localized side effects at the application site. These may include:
- Stinging or burning sensation upon application.
- Mild eye irritation or redness.
- Blurred vision.
- Itching or swelling of the eyelids.
Rare Systemic Side Effects: The systemic effects from antibiotic eye drops are very rare but could include:
- A strange or unpleasant taste in the mouth due to drainage.
- Generalized allergic reactions (e.g., rash, swelling).
Common Oral Antibiotic Side Effects: In contrast, oral antibiotics are known for systemic issues, such as:
- Nausea and vomiting.
- Diarrhea and abdominal cramping.
- Headache and dizziness.

Conclusion

While the prospect of diarrhea from any antibiotic is understandable, the concern that antibiotic eye drops cause diarrhea is largely unfounded. The pharmacological distinction between topical and oral administration is crucial: oral antibiotics affect the entire body, including the digestive system, whereas eye drops have a localized effect with minimal systemic absorption. If you experience any persistent or concerning symptoms while using eye drops, it is essential to consult your ophthalmologist or a healthcare provider. Additionally, mastering the proper application technique can further reduce the small chance of systemic side effects. For more information on proper application, consult authoritative sources like the Glaucoma Research Foundation.

Glaucoma Research Foundation - Eye Drop Techniques

Frequently Asked Questions

No, fluoroquinolone eye drops (e.g., Ofloxacin) are unlikely to cause diarrhea because the systemic absorption from topical application is negligible. While oral fluoroquinolones are known to cause diarrhea, the topical form has a minimal impact on the gut microbiome.

No. Tasting eye drops indicates that some of the medication has drained into your throat via the nasolacrimal duct. However, the total amount absorbed systemically is still very small and insufficient to cause diarrhea.

Oral antibiotics are absorbed through the gastrointestinal tract and have systemic effects throughout the body. In contrast, antibiotic eye drops are applied topically to the eye, with minimal amounts of the medication reaching the systemic circulation.

You can minimize systemic absorption by using a technique called punctal occlusion. Gently press your finger on the inner corner of your eye, near your nose, for a few minutes after administering the drops. This blocks the nasolacrimal duct, reducing drainage into the throat.

The most common side effects are local to the eye and include mild stinging or burning upon application, blurry vision, eye irritation, and redness. These symptoms are typically temporary and resolve shortly after use.

While it's highly improbable that the eye drops are the cause, if you experience diarrhea, it's best to consult a healthcare provider. It may be caused by an unrelated issue or another medication you are taking.

While the small amount that drains naturally is harmless, you should never intentionally swallow antibiotic eye drops. Accidental ingestion of certain non-antibiotic eye drops, such as those containing tetrahydrozoline, can be very dangerous, especially for children.