Why Do Eye Drops Make Your Mouth Taste Bitter?
― The Inner Corner of Your Eye Has a Drain That Leads to Your Nose
Your eyes and your mouth sit far apart, one high on your face and one low. Yet when you put in eye drops, your tongue tastes something bitter. The reason is a tiny hole in the inner corner of each eye. Tears drain through it into the back of your nose every day. Eye drops travel the same road, all the way to your throat.
Your eyes feel tired, so you put in one eye drop. It feels cool, and you blink a few times.
A little later, a bitter taste spreads at the back of your throat. And you didn't swallow anything.
Come to think of it, you get a runny nose after crying, too. Are your eyes, nose and mouth connected somewhere inside your face?
There are only two reasons
Each eye has one tiny hole on the inner-corner side of the upper and lower eyelids. Tears are sucked into these holes and flow through thin tubes to the back of the nose.
The surface of the eye can hold only a tiny amount of liquid. One eye drop is more than that, so the extra spills onto your cheek, and the rest quickly flows down the drain.
Together, these two facts mean most of an eye drop can't stay in the eye long. It runs down from the back of the nose into the throat. There, your tongue notices the bitter ingredient in the drop.
Where do tears go every day?
Tears don't come only when you cry. They keep seeping out a little at a time while you are awake, from the tear gland above the outer part of the eye. Each blink spreads them over the surface of the eye and keeps it from drying out.
If tears keep coming, they have to be disposed of somewhere, or they would overflow. The exit is the punctum (tear point) in the inner corner of the eye. Look at Figure 1. If you gently pull your lower eyelid down in a mirror, you can see a tiny dot on the edge near the inner corner.
Tears that enter a punctum pass through a thin tube called the canaliculus and collect in a small bag called the lacrimal sac. From there they go down the nasolacrimal duct and come out in the space at the back of the nose. That space joins the throat, so the tears are finally swallowed.
The main force driving this flow is blinking. When you close your eyes, the muscles around the eyelids are thought to squeeze and release the sac, pumping the tears along. You get a runny nose after crying because the extra tears flow along this path into your nose.
Even one drop is too much for an eye
Now let's look at the amounts. The tears normally on the surface of the eye are about 7 microlitres. One microlitre is a thousandth of a millilitre. That is about the size of one tiny droplet.
The most an eye can hold, counting the tears already there, is said to be about 30 microlitres. A single commercial eye drop, on the other hand, is about 30 to 50 microlitres. In other words, one drop is bigger than the room left for new liquid. Use the slider in Figure 2 to change the number of drops.
Even with one drop, almost half can't fit and spills out. With two drops, more than 70% overflows. And what does get in doesn't stay long. The blink pump is thought to send most of it to the nose within a few minutes.
That is why many eye medicines say "one drop at a time". Closing your eyes after applying the drop and gently pressing the inner corner with a finger also helps. It blocks the drain, so the medicine stays in the eye longer and less of it flows to the throat. If you keep blinking rapidly instead, the pump works and the medicine is flushed out.
Not every eye drop tastes bitter. You notice bitterness only when the medicine contains a bitter ingredient. You can taste it because the back of the tongue, near the throat, also has taste sensors. Medicine that flows to the back of the nose can also be absorbed into the body through the lining of the nose. Pressing the inner corner is recommended partly to reduce that amount.
If you apply them one right after the other, the second washes out the first, because the room in the eye is so small. In general, waiting about 5 minutes between them is considered best. If your doctor or pharmacist gives you different instructions, follow those.
Summary
The inner corner of the eye has a tear drain that leads to the back of the nose and then the throat. The eye can hold less than one drop, so most of an eye drop takes this road to the throat, and the tongue notices the bitterness.
The bitter taste of eye drops is proof that your eyes, nose and throat
are joined by a single tube.
For more on tears themselves, see "Why do we cry when we cut an onion?". For the back of the nose, see "Why does only one side of your nose get blocked?". For how taste and smell are linked, see "Why does food lose its taste when you have a cold?".
- Look in a mirror in a bright place and gently pull your lower eyelid down with a finger. On the edge near the inner corner you will see a tiny hole like a dot. This is the punctum. The upper eyelid has one too.
- The next time you use eye drops, close your eyes right after and gently press the inner corner for about a minute.
- Compare with a day when you didn't press. Notice how the time before the bitter taste appears, and its strength, differ.
Don't put your finger in your eye or press hard. Use eye drops only as directed, and try this only with the ones you already use.
For those who want to know more ― terms, formulas and links to textbooksEach section is labelled with its level, from lower secondary school science to university courses
- Middle schoolCovered in lower secondary school science
- High schoolCovered in upper secondary school biology or physics
- High school+Advanced high school material, or textbook sidebar topics
- UniversityUniversity-level specialist subjects not taught in high school (ophthalmology, fluid mechanics, pharmacokinetics)
- ResearchNot yet taught even at university as settled fact; being studied by researchers right now
Middle schoolTerms: this phenomenon has names
- Punctum (tear point): the tiny hole on the inner-corner side of the upper and lower eyelids that acts as the exit for tears.
- Lacrimal passage: the name for the whole tear drainage route, from the punctum through the canaliculus, lacrimal sac and nasolacrimal duct to the back of the nose.
- Conjunctival sac: the gap between the inside of the eyelid and the eyeball. Eye drops collect here, and it can hold only a limited amount.
- Nasolacrimal occlusion (lacrimal sac compression): pressing the inner corner after applying a drop to block the lacrimal passage and reduce the amount of medicine that flows to the nose.
Middle schoolHigh schoolChecking with a formula: how much of one drop overflows?
Think of the eye as a small container with a fixed limit. The volume of liquid that comes in doesn't vanish, so whatever doesn't fit overflows. The symbols and units are as follows.
| Symbol | Meaning and unit |
| D | Amount of eye drop applied (microlitres) |
| C | Most the eye can hold (microlitres) |
| T | Amount of tears already present (microlitres) |
| E | Overflow amount (microlitres) |
| Q | Rate at which tears are made (microlitres per minute) |
| In symbols | E ＝ D − （C − T） |
| In words | Overflow ＝ amount applied − (most the eye can hold − amount of tears already there) |
| Where it comes from | Liquid volume doesn't vanish (conservation of volume). Whatever exceeds the available room has nowhere to go but out of the container |
| Volume of one eye drop D (said to be 30–50; here, a middle value) | 40 microlitres |
| Most the eye can hold C | about 30 microlitres |
| Usual amount of tears T | about 7 microlitres |
| Rate at which tears are made Q | about 1.2 microlitres per minute |
| Room for new liquid in the eye (C − T) | 30 − 7 ＝ 23 microlitres |
| Overflow from one drop E | 40 − 23 ＝ 17 microlitres |
| Share of the amount applied that overflows | 17 ÷ 40 ≒ 0.43 |
| Amount when two drops are applied | 40 × 2 ＝ 80 microlitres |
| Overflow from two drops | 80 − 23 ＝ 57 microlitres |
Even with one drop, just over 40% spills, and the second drop is almost entirely wasted. Even the 23 microlitres that do get in mostly leave through the drain in Figure 1 within a few minutes.
| Replacement time ＝ amount of tears ÷ rate of production (T ÷ Q) | 7 ÷ 1.2 ≒ 5.8 minutes |
Even if you do nothing, the tears on the surface of your eye are replaced by fresh ones in about 6 minutes, by this calculation. The surface of the eye is not a still puddle but more like a stream that keeps flowing slowly.
High schoolHigh school+What sets the flow: a pump and a narrow tube
High schoolThe idea that "when the amount coming in and the amount going out balance, the amount held stays constant" is the same idea as the regulation of body fluids you learn in high school biology. Tears are kept at a steady amount because the rate at which they are made balances the rate at which they leave through the punctum (plus evaporation).
High school+The puncta and canaliculi are thin tubes less than 1 millimetre across. In thin tubes, capillary action comes into play, where surface tension draws liquid in. Along with the blink pump, this force is thought to help pull tears into the puncta.
UniversityFlow in thin tubes, and where the medicine goes
By the Hagen–Poiseuille law, the amount of liquid flowing through a thin tube is proportional to the fourth power of the tube's radius. If the radius is halved, the flow drops to one sixteenth. This strong dependence is why nasolacrimal duct obstruction, where tears overflow when the duct narrows with age or inflammation, occurs. Also, only a few percent of an applied drop is said to reach the tissues of the eye, and the fate of the rest is treated in pharmacokinetics as "systemic absorption".
📖 Derivations and further reading: Nasolacrimal duct (Wikipedia, in Japanese) ／ Capillary action (Wikipedia, in Japanese)
ResearchWhat is still not clear
- How the upper and lower puncta share the work: Results differ depending on how it is measured, so it is unclear how much each punctum carries.
- The detailed mechanism of the tear pump: There are several theories about how blinking squeezes and draws in the lacrimal sac, and the debate continues.
- Ways to keep medicine in the eye longer: Researchers are developing formulations that are harder to wash out, such as thickened drops and liquids that turn into a gel.
So the content of this article, too, is "an explanation of what is understood so far". The figures vary with the person and the way of measuring, and typical values are used here.
Links to textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Middle school | Science, year 2: sense organs / volume | The path of tears; the calculation of how much one drop overflows |
| High school | Biology: maintaining the internal environment | Balance between the amount made and the amount leaving |
| High school+ | Physics: surface tension (advanced) | Tears pulled into the punctum by capillary action |
| University | Fluid mechanics, ophthalmology, pharmacokinetics | Hagen–Poiseuille law, nasolacrimal duct obstruction, systemic absorption |
| Research | Ophthalmology, pharmaceutics | Sharing between puncta, the tear pump, ways to keep medicine in the eye |
| ― | Link to daily life | One eye drop at a time; gently press the inner corner afterwards |
- Mishima S, et al. Determination of tear volume and tear flow. Investigative Ophthalmology, 1966.
- 『標準眼科学』 (Standard Ophthalmology), Igaku-Shoin (anatomy of the lacrimal system and drainage of tears)
- General-reader explanations of how to use eye drops correctly, by the Japan Ophthalmologists Association (日本眼科医会), the Japan Pharmaceutical Association (日本薬剤師会) and others
- 鼻涙管 (Nasolacrimal duct) - Wikipedia, in Japanese
- 涙 (Tears) - Wikipedia, in Japanese
*This article is a general-audience science explanation. The figures given are rough estimates to help you understand how things work. For how to use eye drops, follow the product instructions and the advice of your doctor or pharmacist. If eye pain or redness continues, see an eye doctor.