A single eye tearing up usually means something is irritating or blocking that eye specifically. Your eyes produce tears constantly at a low, steady rate to keep the surface moist. But when one eye encounters an irritant, a scratch, a blocked drainage channel, or even chronic dryness, a reflex arc between two cranial nerves triggers the tear gland to ramp up production. That reflex can increase tear output up to 100 times the normal baseline, and if your eye’s drainage system can’t handle the sudden flood, tears spill down your cheek.
Dry Eye Is the Most Counterintuitive Cause
It sounds backward, but dryness is one of the most common reasons a single eye waters nonstop. When one eye isn’t producing enough of the oily or mucus layers that keep your tear film stable, the surface dries out and becomes irritated. Your body responds by flooding that eye with watery reflex tears. These emergency tears lack the right composition to actually lubricate the surface, so the cycle repeats: dryness triggers flooding, the watery tears evaporate or overflow, and the eye dries out again.
This tends to be worse in one eye because tear production and tear film quality aren’t perfectly symmetrical. You might notice it more in dry environments, after long screen sessions, or on windy days. If your tearing eye also feels gritty or stings, dry eye is a strong possibility.
A Blocked Tear Duct
Each eye drains tears through a tiny channel near the inner corner of your eyelid, down into your nose. When that channel (the nasolacrimal duct) gets partially or fully blocked on one side, tears have nowhere to go and pool on the surface. The affected eye stays watery throughout the day regardless of whether anything is irritating it, which distinguishes this from most other causes.
Other signs of a blocked duct include redness, crusting along the eyelids, mucus or pus discharge, and recurring infections like pink eye on the same side. You may also notice painful swelling near the inside corner of the eye, where the duct opening sits. In adults, blockages can develop gradually from chronic inflammation, prior infections, or age-related narrowing. If your eye has been tearing constantly for several days or keeps getting infected, that pattern points toward a drainage problem.
Doctors can test for this by placing a drop of fluorescent dye in the eye. If the dye is still sitting on the surface after five minutes instead of draining away, it suggests a partial or complete obstruction.
Something in Your Eye or a Scratch
A tiny foreign object, even an eyelash or a speck of dust, trapped under the upper eyelid can produce intense tearing in that eye alone. The object scrapes the cornea every time you blink, creating one or more vertical scratches across the eye’s surface. You’ll typically feel a sharp, gritty foreign-body sensation along with redness and sometimes a watery discharge.
A corneal abrasion (a scratch on the clear front surface of the eye) causes the same symptoms even after the object is gone. The damaged nerve endings on the cornea keep firing, triggering the reflex tearing response. Small scratches usually heal within a day or two, but the tearing can be significant while recovery is underway. Contact lens wearers are especially prone to these micro-injuries.
Infection in One Eye
Conjunctivitis (pink eye) often starts in one eye before spreading to the other. The type of infection shapes what you’ll see alongside the tearing:
- Bacterial conjunctivitis produces thick, yellowish or greenish discharge that mats your eyelids together overnight. The eye looks red and may feel swollen or painful.
- Viral conjunctivitis tends to cause a thinner, watery discharge with redness. Certain types, like those linked to herpes simplex, can affect only one eye and may produce blister-like lesions on the skin near the eye.
- Allergic conjunctivitis usually hits both eyes, so if only one eye is involved, an allergic cause is less likely.
Eyelid Position Problems
Your lower eyelid acts like a gutter, channeling tears toward the drainage opening. If that eyelid turns outward (a condition called ectropion), the inner surface becomes exposed to the air and tears can’t flow into the duct properly. The result is persistent tearing, dryness, and irritation on that side. If the eyelid turns inward instead (entropion), the lashes scrape the eye’s surface with every blink, triggering reflex tearing and discomfort.
Both conditions are more common in older adults as the muscles and tendons supporting the eyelid loosen with age. They typically affect one eye more than the other, which is why tearing shows up on just one side.
Nerve-Related Causes
Bell’s palsy, a condition that temporarily weakens or paralyzes the muscles on one side of the face, frequently causes excessive tearing in the affected eye. The weakness makes it difficult to fully close that eyelid, so the eye dries out and triggers reflex tears. At the same time, the muscle that normally pumps tears into the drainage duct isn’t working well, so tears overflow even at normal production levels.
People with Bell’s palsy often need lubricating drops or an eye patch, especially during sleep, to protect the exposed eye from drying out and collecting debris. Tearing from Bell’s palsy usually improves as the nerve recovers, though in some cases people develop a long-term pattern where the eye waters during eating, sometimes called “crocodile tears,” due to misdirected nerve regrowth.
When Tearing Signals Something Serious
Most causes of one-sided tearing are manageable and not urgent. But certain combinations of symptoms need prompt attention. Go to an emergency room if you’ve had trauma to the eye or face and you suddenly lose vision in one or both eyes. You should also seek same-day care if you feel or see a lump or bump on the eye, your vision has changed noticeably, or the tearing is making it hard to see clearly. Constant tearing that persists for more than a few days, especially with recurring infections or increasing pain, warrants a visit to an eye care provider to check for a structural blockage or chronic surface problem.