A cat blinking or squinting with one eye is almost always a sign of pain, irritation, or discomfort in that eye. Cats instinctively guard an injured or irritated eye by partially closing it, and since the problem is usually limited to one side, the blinking looks asymmetrical. While a slow, relaxed blink with both eyes is normal cat communication, repeated or sustained one-eyed blinking is your cat telling you something is wrong.
What One-Eyed Blinking Looks Like
The behavior can range from subtle to obvious. You might notice your cat holding one eye slightly more closed than the other, squinting under bright lights, or repeatedly blinking on one side. Some cats paw at the affected eye or rub their face against furniture. In more serious cases, the eye stays nearly shut, and the cat avoids being touched on that side of the head.
Veterinary pain researchers have developed a standardized tool called the Feline Grimace Scale that scores five facial features to assess pain in cats: ear position, orbital tightening (how much the eye narrows), muzzle tension, whisker position, and head posture. Orbital tightening, the exact thing you’re noticing, is one of the strongest indicators that a cat is experiencing discomfort. If you see it alongside flattened ears, tense whiskers pushed forward, or a lowered head, your cat is likely in significant pain.
Something Stuck in the Eye
The most straightforward explanation is a foreign object. A speck of dust, a loose hair, a bit of litter, or a grass seed can lodge under the eyelid or against the surface of the eye. Organic materials like grass awns are especially common in cats that go outdoors, and they can be surprisingly difficult to spot without magnification. Your cat will typically start blinking suddenly, often within minutes of the irritation starting, and may produce watery, clear discharge from that eye.
If you can see a loose piece of debris, gently flushing the eye with sterile saline can sometimes dislodge it. But even if flushing seems to work, the object may have already scratched the cornea. Foreign bodies can cause corneal ulcers or secondary infections, so a vet visit is still a good idea.
Conjunctivitis
Conjunctivitis, inflammation of the pink tissue lining the eyelids, is one of the most common reasons cats squint or blink on one side. The classic signs are frequent blinking, squinting, redness, a swollen or puffy-looking third eyelid, and discharge. That discharge is an important clue: clear and watery usually points to a viral infection or an environmental irritant like dust or airborne chemicals, while thick, yellow, or green discharge suggests a bacterial infection or a viral infection that has progressed.
The most frequent cause of feline conjunctivitis is infection, particularly with feline herpesvirus, calicivirus, or bacteria like chlamydophila and mycoplasma. Environmental irritants and allergens from outdoor plants can also trigger it. Cats with weakened immune systems, including those infected with feline immunodeficiency virus (FIV) or feline leukemia virus (FeLV), are more susceptible.
Feline Herpesvirus: The Repeat Offender
Feline herpesvirus type 1 (FHV-1) deserves its own mention because it’s the single most common infectious cause of eye problems in cats, and it has a frustrating tendency to come back. After the initial infection, which often happens in kittenhood, the virus goes dormant in nerve cells. It can reactivate weeks, months, or years later, triggered by stress, a change in living situation, illness, or even pregnancy and nursing.
When the virus flares up, it travels down nerve fibers to the surface tissues of the eye. This can cause anything from mild redness and watering to painful corneal ulcers with a distinctive branching pattern that vets recognize on examination. Recurrent flare-ups tend to be less severe than the original infection, and respiratory symptoms like sneezing are often absent the second time around. The eye symptoms, squinting, blinking, tearing, and sometimes cloudy spots on the cornea, take center stage instead.
Treatment for herpesvirus flare-ups focuses on antiviral medications (typically applied as eye drops or given orally for about three weeks), reducing stress in the cat’s environment, and sometimes adding antibiotics to prevent secondary bacterial infections from taking hold in the damaged tissue. These episodes tend to resolve with treatment but can recur throughout a cat’s life.
Corneal Ulcers
A corneal ulcer is essentially a wound on the clear surface of the eye. It can result from a scratch, a foreign body, dry eye, or a herpesvirus flare-up. Cats with corneal ulcers often hold the affected eye tightly shut rather than just blinking, and they’re clearly in pain. You may notice excessive tearing, cloudiness in the eye, or visible redness.
Corneal ulcers are diagnosed using a special dye that highlights damaged areas on the eye’s surface. Small ulcers can sometimes merge into larger ones if untreated, and herpesvirus ulcers in particular can linger for extended periods, causing swelling and new blood vessel growth into the cornea. This is one of the situations where prompt treatment makes a real difference in outcome, since a shallow ulcer that heals cleanly is far better than a deep one that scars.
The Third Eyelid
Cats have a third eyelid, a thin membrane tucked in the inner corner of each eye that sweeps across the surface to protect and moisten it. You don’t normally see much of it when a cat is awake and alert. If you notice a white or pinkish film partially covering one eye, it means the third eyelid is elevated or protruding. This can happen because the eye is painful and has retracted slightly into the socket (pulling the third eyelid forward), because the third eyelid’s own gland has prolapsed, or because of inflammation from conjunctivitis or another condition.
A visible third eyelid on one side, combined with blinking or squinting, strongly suggests that eye needs attention. In some cases, the gland of the third eyelid prolapses spontaneously without any other visible eye problem, appearing as a small pink mass in the corner of the eye.
Flat-Faced Breeds Are More Vulnerable
Persians, Himalayans, Burmese, and Exotic Shorthairs have shortened skulls that change the geometry of their eye sockets. Their eyes sit more prominently, their tear ducts drain less efficiently, and their eyelids may not close completely over the corneal surface. All of this makes them more prone to corneal irritation, chronic tearing, and infections that can cause one-sided squinting. If you have a flat-faced breed, occasional eye issues are unfortunately part of the territory, and regular monitoring matters.
What the Discharge Tells You
Pay attention to what’s coming out of the affected eye, because it helps narrow down the cause:
- Clear and watery: Usually an early-stage viral infection, mild irritation, or an allergen. This is the least alarming type, but it still warrants watching.
- Yellow or green and thick: Indicates a bacterial component, either as the primary cause or as a secondary infection layered on top of a viral one. This generally needs treatment.
- Changing from clear to colored: A condition that started as viral or irritant-based may be progressing. If the discharge shifts from watery to mucousy over a day or two, the situation is getting worse, not better.
When It Needs Urgent Attention
Some one-eyed blinking can wait a day for a regular vet appointment. Some can’t. Treat it as urgent if the eye appears cloudy or has changed color, if there’s obvious swelling or bulging, if your cat can’t open the eye at all, if you can see blood in or around the eye, or if your cat’s vision seems impaired (bumping into things, misjudging jumps). A red, cloudy, or closed eye with discharge is the combination that most warrants same-day care, since conditions like deep corneal ulcers or glaucoma can threaten vision within hours.
If your cat is blinking on one side but the eye looks relatively normal, there’s no colored discharge, and the behavior just started, it’s reasonable to observe for 12 to 24 hours. A piece of dust or a minor irritant can resolve on its own. But if the squinting persists beyond a day, or if any discharge, redness, or swelling develops, that waiting period is over.