Tearing from only one eye is medically known as unilateral epiphora, signifying an imbalance in the eye’s delicate tear system. The eye constantly produces tears to keep the surface lubricated, clear, and protected. This production must be matched by efficient drainage to prevent overflow. When excessive tearing occurs in just one eye, it signals a localized problem causing tears to be overproduced or preventing correct drainage.
Temporary and External Irritants
The most common reasons for a sudden, one-sided watery eye are transient irritants that trigger a protective reflex to flush the surface clean with a surge of tears. This reflex quickly dilutes and removes the offending substance.
Environmental factors like cold wind, strong air conditioning, or smoke can cause intense, short-lived tearing in the most exposed eye. Similarly, the entry of a small foreign body, such as dust or a stray eyelash, initiates this rapid flushing mechanism. Because the irritation is confined to one eye, the tearing remains unilateral.
Chemical exposure from strong odors, such as onion fumes or cleaning products, also causes a reflexive tearing response. This temporary overproduction of tears typically subsides shortly after the irritant is removed and does not indicate a chronic medical condition.
Issues with the Tear Drainage System
When unilateral epiphora persists, the problem often involves a mechanical failure in the eye’s drainage system rather than tear overproduction. Tears normally exit the eye through two small openings called puncta, located on the inner corners of the eyelids. They then travel through tiny tubes (canaliculi) into the lacrimal sac and finally down the nasolacrimal duct into the nasal cavity.
A physical obstruction anywhere along this path prevents tears from draining, causing them to back up and spill over the eyelid. Since blockages are frequently unilateral, the overflow is restricted to the affected eye.
In adults, the most common site of obstruction is the nasolacrimal duct, leading to acquired nasolacrimal duct obstruction. This blockage can result from age-related narrowing, chronic inflammation, or scarring from previous injury. Narrowing of the puncta, known as punctal stenosis, also restricts drainage and is a common cause of unilateral tearing in older individuals.
Infections and Inflammatory Conditions
Tearing from one eye can signal an underlying infection, inflammation, or structural change to the eyelid. Inflammation of the conjunctiva (unilateral conjunctivitis or “pink eye”) triggers irritation that causes the lacrimal gland to produce excess tears. Similarly, blepharitis, which is chronic inflammation of the eyelid margins, stimulates reflex tearing due to debris and irritation.
Structural problems with the eyelid can also redirect tears away from the drainage system. Entropion is a condition where the eyelid turns inward, causing eyelashes to rub against the cornea and trigger reflex tearing. Conversely, ectropion is an outward turning of the lower eyelid, which misdirects the punctum and prevents proper drainage.
Paradoxically, chronic dry eye can cause one-sided excessive tearing. When the eye surface becomes severely dry, it triggers a reflex flood of watery tears that overwhelms the normal drainage capacity, leading to temporary epiphora. If the dry eye or irritation is more pronounced in one eye, the reflex tearing will also be unilateral.
When to Consult a Professional
While many instances of unilateral tearing are temporary, certain signs suggest the need for a professional evaluation. If the excessive watering persists for more than a few days without a clear, transient cause, or lasts longer than a few weeks, it often points toward a functional or mechanical drainage problem that may require intervention.
A doctor should be consulted immediately if the watery eye is accompanied by pain, sudden changes in vision, pronounced redness of the eye or eyelid, or sensitivity to light. The presence of a thick, yellow, or greenish discharge can signal a bacterial infection that requires prescription treatment.
An unresolved foreign body sensation or any instance of trauma to the eye that results in persistent watering also warrants medical attention. A thorough examination can determine if the tearing is due to a simple irritation, a chronic inflammation, or a blockage in the lacrimal system.