When only one eye waters persistently, it almost always points to a localized problem on that side rather than a systemic issue like allergies. Unilateral tearing is most commonly caused by a partial or full blockage in the tear drainage system, a foreign body or scratch on the eye’s surface, or irritation from an eyelid that isn’t sitting in its normal position. About 75% of tear duct blockages affect only one side, which is why single-eye watering is such a hallmark of drainage problems.
How Tear Drainage Works
Your eyes constantly produce a thin film of tears to stay lubricated. Normally, each blink pushes tears toward two tiny openings (called puncta) at the inner corner of your eye. From there, tears flow through narrow channels into the nasolacrimal duct, which empties into your nose. That’s why your nose runs when you cry.
When anything disrupts this system on one side, tears have nowhere to go and spill down your cheek. The eye itself may be producing a perfectly normal amount of tears. The problem is that they can’t drain.
Blocked Tear Duct
A blocked tear duct is the single most common reason for persistent watering in one eye. In adults, the blockage can develop gradually as the drainage openings narrow with age, or it can result from a past infection, facial injury, or even tiny particles of dirt or skin cells lodged in the duct. Rarely, a growth in the nose or along the drainage pathway causes the obstruction.
Beyond the constant watering, you may notice redness in the white of the eye, crusty eyelids (especially in the morning), blurred vision, or mucus discharge. Some people get repeated bouts of pink eye on the affected side because stagnant tears become a breeding ground for bacteria. If you press gently on the inner corner of the eye and see milky or yellowish fluid seep out, that strongly suggests the duct is blocked.
Left untreated, a blocked duct can progress to a lacrimal sac infection called dacryocystitis. In its mild form, you’ll have persistent tearing and occasional discharge. In its acute form, the area below the inner corner of your eye becomes red, swollen, and painful, sometimes with pus draining onto the skin. Acute infections develop over hours to days and typically need antibiotic treatment before the underlying blockage can be addressed.
Corneal Scratch or Foreign Body
A tiny scratch on the clear surface of your eye (the cornea) triggers intense, one-sided watering that comes on suddenly. The cornea has hundreds of times more pain receptors than your skin, so even a microscopic abrasion feels like something large is stuck in your eye. You’ll typically also have redness, light sensitivity, and hazy vision on that side.
Common culprits include a grain of sand, a stray eyelash, a contact lens edge, or even rubbing your eye too aggressively. Most small corneal scratches heal within one to three days, but deeper ones can become infected and need treatment. If you feel a sharp, gritty sensation that started suddenly, resist the urge to rub the eye, as that can make the scratch worse.
Eyelid Position Problems
Your eyelids need to sit flush against the eyeball for tears to spread evenly and drain properly. Two conditions can throw this off on one side. Ectropion is when the lower lid turns outward, pulling away from the eye so the upper and lower lids can no longer close tightly. Tears aren’t swept toward the drainage openings and instead roll down your face. Entropion is the opposite: the lid turns inward, pushing lashes against the eyeball. The constant rubbing irritates the surface and triggers heavy reflex tearing.
Both conditions cause watering, redness, and a gritty foreign-body sensation. They’re more common in older adults as the tissues supporting the eyelid loosen, and they frequently affect just one side.
The Dry Eye Paradox
This one surprises most people: a watery eye can actually be a dry eye. When the surface of one eye becomes too dry or irritated, it triggers a reflex arc between the sensory nerves in the cornea and the nerves controlling tear production. The eye responds by flooding itself with emergency tears. These reflex tears are watery and thin, lacking the oily layer that keeps normal tears from evaporating, so they overflow without actually solving the dryness.
If you spend long hours staring at a screen, work in air-conditioned or heated environments, or wear contact lenses, one eye may dry out faster than the other. The result is paradoxical tearing on that side. The watering often comes in waves, alternating with periods where the eye feels scratchy or dry.
Facial Nerve Weakness
Blinking doesn’t just spread tears; it actively pumps them into the drainage system. Each time you blink, a small muscle around your eye compresses the tear channels and creates a vacuum that pulls tears in. When the facial nerve on one side is weakened, as in Bell’s palsy, this pumping mechanism breaks down.
The weakened muscle can’t close the eyelid completely, a condition called lagophthalmos. Tears pool on the surface because the pump isn’t working, and the exposed cornea dries out between incomplete blinks, triggering even more reflex tearing. If your one-sided watering came on alongside facial drooping, difficulty closing the eye, or a change in taste, the facial nerve is likely involved.
How a Blocked Duct Is Diagnosed
An eye doctor can usually identify the cause of one-sided tearing with a physical exam. They’ll look at your eyelid position, check for surface scratches with a special dye, and press on the inner corner of the eye to see if anything comes back through the drainage openings.
If a duct blockage is suspected, one straightforward test involves placing fluorescein dye in the eye and checking whether it appears in the nose after several minutes. If the dye stays in the eye beyond five minutes, that signals an obstruction somewhere in the drainage path. A syringing test, where saline is gently flushed through the duct, can pinpoint exactly where the blockage sits.
Treatment Options
Treatment depends entirely on the cause. Corneal scratches typically heal on their own with lubricating drops and time. Dry eye responds to artificial tears, and in some cases, preserving more of your natural tears with tiny plugs placed in the drainage openings. Eyelid malpositions are corrected with a minor surgical procedure to reposition the lid.
For a confirmed duct blockage, the standard surgical fix is called dacryocystorhinostomy, which creates a new drainage pathway between the tear sac and the inside of the nose. It’s one of the most reliable procedures in ophthalmology, with success rates consistently between 80% and 95%. One large study of 769 patients found that nearly 99% achieved an open drainage pathway after surgery, and about 82% had complete resolution of their watering. A small silicone tube is sometimes placed during surgery to keep the new pathway open while it heals, and it’s removed weeks to months later. Recovery is longer with the external approach (through a small skin incision) compared to the endoscopic version done entirely through the nose.
Signs That Need Prompt Attention
Most causes of one-sided tearing are not emergencies, but certain combinations of symptoms warrant a same-day or next-day visit. Pain in or around the eye, visible swelling near the inner corner, a lump you can see or feel, blood-tinged discharge when pressing on the tear sac area, or any change in vision all call for professional evaluation. If the watering started after a facial injury, or if you suddenly lose vision in the affected eye, that’s an emergency room situation. Persistent one-sided watering that lasts more than a week or two, even without pain, is worth getting checked to rule out a progressive blockage before it leads to infection.