How Eye Drops Enter Your Body's System
When you place a drop of medication into your eye, it doesn't just stay in the eye itself. A significant portion of the liquid and its active ingredients can travel beyond the ocular surface and into your systemic circulation. This journey is primarily facilitated by the nasolacrimal drainage system. The following steps illustrate the process:
- The Tear Drain: Located in the inner corner of each eye are two tiny openings called puncta. Normally, these openings act as drains for excess tears, which flow through small canals into the nasolacrimal duct.
- The Nasal Connection: The nasolacrimal duct connects the eye to the nasal cavity. This is why your nose runs when you cry and why some people can taste their eye drops after instillation. The medication-infused tear fluid drains into the nose and then into the throat, where it is swallowed.
- Systemic Absorption: The inner lining of the nasal and pharyngeal cavities is highly vascular, meaning it contains many tiny blood vessels. This rich blood supply allows for the rapid absorption of the active medication directly into the bloodstream. This absorption route is particularly concerning because it bypasses the liver's first-pass metabolism, which normally helps to process and detoxify ingested substances. Bypassing this process can lead to a more potent systemic effect from the medication.
The Link Between Eye Drops and Digestive Issues
Once the active ingredient from eye drops enters the systemic circulation, it can affect various organs, including those in the digestive tract. The severity and type of digestive side effects depend heavily on the specific medication, its concentration, and individual sensitivity.
- Nausea and Vomiting: Certain medications, when absorbed systemically, can trigger nausea and vomiting. For example, some over-the-counter (OTC) eye drops contain the decongestant tetrahydrozoline. While an urban myth suggests this causes diarrhea, ingesting it can actually lead to severe symptoms, including nausea, vomiting, and other life-threatening effects. Prescription eye drops, like beta-blockers for glaucoma, can also cause systemic side effects, although digestive issues are less common than cardiovascular effects.
- Diarrhea: The notion that eye drops cause diarrhea is a persistent myth, stemming partly from the misinformation surrounding the dangers of consuming OTC redness-relieving drops. However, some very specific, and now rare, prescription drugs, like echothiophate iodide, have historically been linked to diarrhea as a side effect. It's crucial to understand that this is not a general effect of most eye drops and is not the primary risk of systemic absorption.
- Lack of Effect: In some cases, the opposite happens. The acidic environment of the stomach or digestive enzymes can inactivate the medication, making it less effective than intended. This is more about an issue of efficacy rather than a digestive side effect.
A Comparison of Absorption Routes
| Feature | Ocular Absorption | Oral Ingestion (from drainage) |
|---|---|---|
| Primary Route | Permeation across the cornea and conjunctiva to target eye tissues. | Drainage through nasolacrimal duct, then swallowing. |
| Effect on Digestive System | Minimal direct impact; side effects are secondary to systemic effects. | Can cause direct irritation, nausea, vomiting, or other symptoms due to systemic absorption. |
| Bypasses Liver? | No, absorbed drug enters systemic circulation directly. | Yes, a significant amount of the drug is absorbed through nasal mucosa, bypassing first-pass liver metabolism. |
| Bioavailability | Generally low for medication to reach the intraocular tissues (less than 5%). | Can be significant, depending on the drug and the amount absorbed through the nasal mucosa. |
| Associated Risks | Local side effects like irritation, redness, blurred vision. | Potential for more pronounced systemic side effects due to increased bioavailability. |
How to Minimize Systemic Absorption and Digestive Side Effects
Since the swallowed portion of the eye drop is the primary cause of potential digestive side effects, minimizing this absorption is key. Following proper administration techniques can significantly reduce the risk.
- Use the right amount: Instill only one drop per eye at a time. The conjunctival sac can only hold about 10 μL, while most commercial drops are 25–50 μL, so any excess will be wasted and more likely to drain.
- Block the drain: Immediately after instilling the drop, close your eye gently and press with your index finger against the inner corner of your eyelid, near your nose. This technique, called nasolacrimal occlusion, blocks the tear duct temporarily and prevents the medication from draining into your throat.
- Keep your eyes closed: Keep your eyes gently closed for 2–3 minutes after applying the drops. Avoid blinking or squeezing your eyelids, as this can force the drop out.
- Wipe excess: Use a clean tissue to blot any excess solution that flows out of the eye.
- Wait between drops: If you need to use more than one type of eye drop, wait at least 5–10 minutes between applications. This prevents the second drop from washing out the first.
By carefully following these steps, you can help ensure that the medication is primarily absorbed where it's needed—in the eye—while reducing the likelihood of systemic side effects, including those affecting your digestive system. For more detailed instructions, the American Academy of Ophthalmology offers a helpful guide on how to properly put in eye drops.(https://www.aao.org/eye-health/treatments/how-to-put-in-eye-drops)
Conclusion
While eye drops are designed for local application, the interconnected physiology of the eyes, nose, and throat means that systemic absorption is a definite possibility. This route, primarily via the nasolacrimal duct, can lead to medications reaching the digestive system and causing side effects like nausea or vomiting, depending on the specific drug. The persistent urban legend about eye drops causing diarrhea is largely unfounded, particularly concerning OTC products, and misrepresents the true danger of accidental ingestion, which can be much more severe. By mastering proper administration techniques, such as nasolacrimal occlusion and minimizing excess runoff, patients can significantly reduce the risk of unwanted systemic and digestive issues, ensuring that the medication works safely and effectively for its intended purpose.
