Eye drops can cause diarrhea, and the explanation is more straightforward than most people expect. When you put a drop in your eye, much of the liquid drains through tiny channels into your nose and throat, where the active ingredient gets absorbed directly into your bloodstream. From there, the drug can affect tissues throughout your body, including your gut. The phenomenon is well-documented in medical literature going back decades, though it remains underappreciated by both patients and clinicians.
How Eye Drops Reach Your Digestive System
Your eyes are connected to your nose by a pair of narrow tubes called the nasolacrimal ducts. Every time you blink after applying an eye drop, the liquid is pumped down through these ducts and into the nasal cavity. From the nasal lining, drug molecules can pass directly into the bloodstream through the mucosa, which is thin and highly absorbent. Whatever doesn’t get absorbed in the nose continues down the back of your throat, where you swallow it, and it enters your digestive tract the same way any oral medication would.
This is not a minor leak. Studies on ocular drug delivery show that the concentration of active ingredients in eye drops is usually much higher than what the eye itself can absorb. The eye’s surface holds only a tiny volume of fluid, so a standard drop delivers far more medication than the eye needs. The excess has to go somewhere, and most of it ends up draining into the nose and throat.
Substances can be absorbed through the conjunctival sac itself, through the nasal mucosa as the fluid drains, or through ingestion after the drop reaches the back of the throat.1PubMed Central. Absorption of Toxicants from the Ocular Surface: Potential Applications in Toxicology The nasal route is particularly efficient because the nasal lining has rich blood supply and high permeability, meaning drugs absorbed there bypass the liver’s first-pass metabolism and enter general circulation at relatively high concentrations.2PubMed Central. Systemic side effects of eye drops: a pharmacokinetic perspective This is the same reason nasal sprays work quickly for pain relief or allergy symptoms. The difference is that nobody intends for their eye drops to act like a nasal spray, but that is functionally what happens.
Which Eye Drops Are Known to Cause Digestive Side Effects
Not all eye drops carry the same risk for gut-related side effects. The ones most commonly linked to diarrhea and other gastrointestinal symptoms fall into a few recognizable categories.
Prostaglandin analogs, such as latanoprost, are widely prescribed for glaucoma. These drugs lower eye pressure by increasing fluid outflow, but prostaglandins are also powerful regulators of inflammation and smooth muscle contraction throughout the body, including in the intestines. A documented case report described a patient with Crohn’s disease whose condition flared after starting latanoprost eye drops. The authors concluded that even the small amount absorbed systemically from a topical eye drop was enough to promote a relapse of the intestinal disease.3PubMed Central. The role of latanoprost in an inflammatory bowel disease flare For someone without a pre-existing bowel condition, the effect might be milder, perhaps just loose stools or cramping, but the mechanism is the same: prostaglandins stimulate gut motility and fluid secretion.
Cholinesterase inhibitors are another class with a long history of GI side effects from ocular use. These drugs, once commonly used in glaucoma treatment, work by inhibiting an enzyme that breaks down acetylcholine, a chemical messenger involved in muscle contraction. When even small quantities reach the bloodstream and act on the smooth muscle of the intestines, the result can be increased motility, cramping, and diarrhea. A report published in the New England Journal of Medicine as far back as 1964 described diarrhea as the first symptom of systemic toxicity from echothiophate iodide eye drops.4New England Journal of Medicine. Eye Drops and Diarrhea While these specific drops are less commonly prescribed today, the case illustrates how seriously eye-drop-related GI symptoms have been documented.
Beta-blockers like timolol, another common glaucoma drug, are better known for systemic effects on the heart and lungs, but they can also influence gut function. Beta-adrenergic receptors are present in the gastrointestinal tract, and blocking them can alter motility patterns. Nausea and abdominal discomfort are listed among the recognized systemic side effects of topical beta-blocker eye drops.
Even antibiotic and anti-inflammatory eye drops can occasionally trigger diarrhea, particularly when used frequently or over longer periods. The small amounts swallowed could, in theory, affect gut flora in the same way that oral antibiotics do, though this is less well-studied for topical ocular doses.
Preservatives and “Inactive” Ingredients
The active drug is not the only thing in the bottle that can cause problems. Most multi-dose eye drops contain preservatives to prevent bacterial contamination, and these so-called inactive ingredients are not biologically inert once they enter your body.
Benzalkonium chloride (BAK) is the most commonly used preservative in ophthalmic preparations. It is a surfactant, meaning it disrupts cell membranes, which is how it kills bacteria. On the eye surface, BAK is known to cause irritation and inflammation, and there is growing concern about what happens when it enters the bloodstream. A recent review highlighted that BAK and other excipients in commonly used artificial tears can enter the systemic circulation and contribute to inflammation beyond the eye.5Advances in Ophthalmology & Visual System. Blood-based inflammatory biomarkers: An overlooked tool for personalizing dry eye therapy – and the potential systemic effects of eye drop inactive ingredients For someone who uses artificial tears multiple times a day, the cumulative exposure to preservatives is not trivial.
Other additives like boric acid, EDTA, and various buffering agents also make the trip from eye to nose to gut. While each individual exposure may be small, people who use several different eye drops on a daily schedule, which is common in glaucoma management, are dosing themselves repeatedly throughout the day. The combination of active drugs and preservatives from multiple bottles adds up. If you are experiencing unexplained GI symptoms and you use eye drops regularly, the preservatives deserve consideration alongside the active ingredients.
Who Is Most at Risk
Some people are more vulnerable to systemic effects from eye drops than others. Understanding who falls into this category can help explain why one person has no trouble while another develops noticeable symptoms from the same product.
Children are at the top of the risk list. Their smaller body mass means that a standard-sized eye drop delivers a proportionally larger dose per kilogram. Their metabolic pathways for clearing drugs are also less mature. In cases where children have accidentally ingested over-the-counter eye drops containing tetrahydrozoline (the active ingredient in many redness-relief products), the consequences have been severe, including extreme drowsiness, dangerously slow heart rate, and breathing difficulties.6PubMed Central. Unintentional pediatric ophthalmic tetrahydrozoline ingestion: case files of the medical toxicology fellowship at the University of California, San Francisco While these cases involve swallowing the drops rather than using them in the eyes, they underscore how potent these small bottles can be for a small body. Even normal ophthalmic use in young children should be approached with awareness of systemic risk.
People with pre-existing inflammatory bowel disease are another group that should pay attention. As the latanoprost case report demonstrated, prostaglandin-containing eye drops can trigger flares in patients with Crohn’s disease or ulcerative colitis.3PubMed Central. The role of latanoprost in an inflammatory bowel disease flare The gut in these conditions is already primed for inflammation, and even a small systemic dose of a pro-inflammatory mediator can tip the balance.
Older adults are also at heightened risk, though for different reasons. They are more likely to be on multiple medications, increasing the chance of drug interactions. They are more likely to have reduced liver and kidney function, slowing the clearance of absorbed drugs. And they are the population most likely to use glaucoma medications long-term, meaning their exposure is chronic rather than occasional. Eye drops are often missing from their medication lists during doctor visits, which makes it harder to connect the dots between a new GI complaint and a long-standing ophthalmic prescription.
People who take multiple eye drops, whether for glaucoma, allergies, dry eye, or some combination, also accumulate more systemic exposure. Each additional drop is another dose entering the nasolacrimal system, and the timing of drops often means one is applied before the previous one has fully cleared.
Simple Techniques to Minimize Absorption
The good news is that you can significantly reduce how much of an eye drop enters your bloodstream by using it more carefully. Two techniques stand out in the research, and both are easy to do at home.
The first is nasolacrimal occlusion, which simply means pressing a finger against the inner corner of your eye, near the bridge of your nose, right after applying the drop. Holding gentle pressure there for one to two minutes physically blocks the opening of the nasolacrimal duct, keeping the medication on the eye surface where it belongs instead of letting it drain into the nose. A review of the literature found that this technique improves how much medication the eye absorbs while discouraging systemic absorption.7PubMed Central. The importance of eyelid closure and nasolacrimal occlusion following the ocular instillation of topical glaucoma medications, and the need for the universal inclusion of one of these techniques in all patient treatments and clinical studies In other words, more drug reaches the eye, and less reaches your gut and bloodstream. That is a win on both fronts.
The second technique is simply keeping your eyelids gently closed for one to two minutes after placing the drop, without squeezing them shut or blinking rapidly. Blinking acts as a pump that pushes fluid toward the drainage ducts, so staying still with eyes closed lets the drop sit on the eye longer.
Despite how effective these methods are, they are often performed poorly or not at all. One study specifically noted that while nasolacrimal occlusion is effective at reducing systemic absorption, patients find it difficult and frequently do it incorrectly.8PubMed. New technique to reduce systemic side effects of timolol eye drops: The tissue press method-Cross-over clinical trial Part of the problem is that most eye care providers do not teach these techniques. Patients leave the office with a prescription and very little instruction on how to use it safely.
Beyond finger pressure and eyelid closure, a few other practical steps help. Wiping away any excess liquid that pools on the skin around your eye prevents it from being absorbed through the skin or running into the eye later and draining. If you use more than one type of eye drop, waiting at least five minutes between drops gives each one time to absorb locally rather than washing the previous drop into the drainage system. And if you have the option, preservative-free formulations eliminate at least one source of unwanted systemic exposure.
Why This Connection Gets Missed
One of the most frustrating aspects of eye-drop-related GI symptoms is how rarely they are recognized for what they are. A patient who develops diarrhea after starting a new glaucoma medication may never connect the two events. Their ophthalmologist may not ask about GI symptoms, and their primary care doctor may not think to ask about eye drops.
This disconnect has been noted in the medical literature for decades. A widely cited paper pointed out that systemic complications from eye drops are frequently left out of a patient’s drug history and overlooked when doctors are trying to figure out the cause of a new adverse reaction.9JAMA Internal Medicine. Systemic Effects of Eye Drops Eye drops occupy a strange mental category for most people: they feel local, topical, and harmless, more like a rinse than a real medication. That perception makes it easy to forget to mention them to a doctor or pharmacist, and easy for a clinician to overlook them as a cause of new symptoms.
The problem is compounded by the way medical specialties are siloed. The ophthalmologist manages the eyes. The gastroenterologist manages the gut. Neither may routinely consider the other’s domain. If you are experiencing new or worsening diarrhea and you use eye drops of any kind, it is worth mentioning the drops to whichever doctor you see about the GI issue. Bring the bottles if you can, including over-the-counter drops, since those are the ones most likely to be forgotten.
Over-the-Counter Drops Deserve Scrutiny Too
When people think about side effects from eye drops, they tend to think about prescription medications. But some of the most widely used over-the-counter products carry their own risks, particularly redness-relief drops and certain artificial tears.
Redness-relief drops typically contain vasoconstrictors like tetrahydrozoline or naphazoline. These drugs constrict blood vessels to make eyes look whiter. When absorbed systemically, they act on blood vessels and the nervous system elsewhere in the body. The primary concerns with these drops are cardiovascular, including changes in blood pressure and heart rate, but GI effects are part of the broader systemic picture. In children, as noted earlier, even small ingested amounts can cause alarming symptoms.6PubMed Central. Unintentional pediatric ophthalmic tetrahydrozoline ingestion: case files of the medical toxicology fellowship at the University of California, San Francisco
Artificial tears might seem completely benign, and preservative-free versions largely are, but the preserved varieties that most people buy are applied multiple times per day. Each application sends a small amount of preservative through the nasolacrimal system and into the body. For someone using preserved artificial tears six or eight times a day for chronic dry eye, the total daily preservative load is real. Switching to a preservative-free option eliminates that exposure entirely and, in some patients, has been associated with improvement in non-ocular inflammatory symptoms as well.5Advances in Ophthalmology & Visual System. Blood-based inflammatory biomarkers: An overlooked tool for personalizing dry eye therapy – and the potential systemic effects of eye drop inactive ingredients
Antihistamine drops used for allergy relief, such as ketotifen, are generally well tolerated but can occasionally cause nausea and stomach discomfort as listed side effects. The doses are small, but for people who are sensitive to antihistamines or who combine the eye drops with oral antihistamines, the additive effect is worth considering.
What to Do If You Suspect Your Eye Drops Are Causing GI Symptoms
If you notice new diarrhea, abdominal cramping, or nausea that coincides with starting a new eye drop or increasing the frequency of one you already use, do not stop a prescribed medication without talking to the prescribing doctor. Abruptly stopping glaucoma drops, for example, can allow eye pressure to spike. But do make the connection known to your provider, because there are often alternatives.
For glaucoma, there are multiple drug classes, and switching from a prostaglandin analog to a different category of pressure-lowering drop may resolve GI symptoms while still protecting your vision. For dry eye, moving from a preserved to a preservative-free artificial tear is a simple change with no downside. For allergy drops, adjusting the frequency or switching to a mast cell stabilizer instead of an antihistamine may help.
In the meantime, adopting the finger-pressure technique after every drop can reduce systemic absorption enough to lessen or eliminate the problem. It costs nothing, takes about a minute, and benefits your eyes as well as the rest of your body. Few medical interventions offer that kind of dual benefit for so little effort.7PubMed Central. The importance of eyelid closure and nasolacrimal occlusion following the ocular instillation of topical glaucoma medications, and the need for the universal inclusion of one of these techniques in all patient treatments and clinical studies
Keep a list of every eye drop you use, including brand name and active ingredient, and treat it like part of your medication list. When you see any doctor, not just your eye doctor, hand over that list along with your other prescriptions. The more information your providers have, the less likely an eye-drop side effect will be chalked up to something else entirely.