A cat that keeps blinking or squinting one eye is almost always signaling pain or irritation in that eye. Unlike a casual slow blink, persistent one-sided squinting (called blepharospasm) is a protective reflex triggered by something affecting the eye’s surface or interior. The cause can range from a speck of dust trapped under the eyelid to a corneal ulcer or viral infection, and some of these conditions can threaten your cat’s vision within hours if left untreated.
What One-Eye Blinking Actually Means
Cats don’t squint one eye for fun. When only one eye is affected, the squinting is a pain response. The muscles around the eye clamp down to protect it, the same way you’d reflexively shut your eye if something flew into it. You might also notice your cat pawing at the eye, increased tearing, or the third eyelid (a translucent inner membrane that slides across the eye for protection) partially covering the affected side.
Because the squinting is driven by discomfort rather than a specific disease, it shows up across a wide range of eye problems. The key is figuring out which one.
Foreign Objects and Scratches
The simplest explanation is something stuck in or near the eye. Plant material, grass seeds, dirt, and dust particles are the most common culprits. A small piece of debris trapped under the eyelid acts like sandpaper against the cornea every time your cat blinks, causing immediate squinting and watery discharge. Cats that go outdoors or live in dusty environments are especially prone.
Scratches are equally common. A swipe from another cat during play or a fight, a brush against a branch, or even an accidental poke from a piece of furniture can scrape the cornea. These injuries are painful and cause the same one-sided squinting pattern. Minor scratches sometimes heal on their own within a day or two, but deeper ones can progress to corneal ulcers if bacteria move in.
Corneal Ulcers
A corneal ulcer is an open sore on the clear surface of the eye, and it’s one of the more serious reasons a cat squints on one side. Causes include trauma from scratches and fights, ingrown eyelashes, chemical exposure, and infection. The most frequent cause, according to Cornell University’s College of Veterinary Medicine, is recurrent infection with feline herpesvirus.
Beyond squinting, signs of a corneal ulcer include discharge seeping from the eye, cloudiness over the cornea, and sensitivity to bright light. Your cat may rub at the affected eye and seem to have trouble seeing on that side. Ulcers can worsen quickly. Even waiting a few hours can mean the difference between successful treatment and irreversible damage, so this is one condition where speed matters.
A vet diagnoses corneal ulcers using a fluorescein stain, a harmless orange dye dropped onto the eye’s surface. The dye sticks to damaged tissue and glows green under a blue light, revealing the ulcer’s exact size and depth. This test is quick and painless.
Conjunctivitis
Conjunctivitis, or inflammation of the tissue lining the eyelids, is another frequent cause. It produces a noticeable discharge that can be either clear and watery or thick and dark-colored, depending on whether the infection is viral or bacterial. The affected eye looks red and puffy, and your cat may hold it partially shut.
The most common triggers are herpesvirus, calicivirus, and two types of bacteria: chlamydophila and mycoplasma. Most cats already carry these organisms, and the inflammation is actually the immune system’s reaction to them rather than direct damage from the microbe itself. Conjunctivitis can affect one or both eyes, but when it starts on one side, the squinting looks exactly like the pattern you’re noticing.
Feline Herpesvirus: The Recurring Culprit
Feline herpesvirus deserves its own mention because it’s behind so many cat eye problems. Most cats pick it up when they’re very young. The virus never fully leaves the body. Instead, it hides in nerve tissue and can reactivate later in life, especially during periods of stress or illness. Virtually all cats exposed to the virus become persistently infected, and roughly half of them will experience a flare-up at some point.
During a primary infection, you’ll typically see conjunctivitis alongside upper respiratory symptoms like sneezing and nasal discharge. The eye involvement is usually bilateral, meaning both eyes are affected, with redness, swelling, and watery discharge appearing after an incubation period of two to six days.
Recurrent flare-ups, however, can be one-sided. The virus may attack the corneal surface, creating distinctive branching (dendritic) ulcers that are considered a hallmark of herpesvirus infection. These ulcers can persist for weeks and are painful enough to cause obvious squinting. If your cat has repeated episodes of one-eye squinting separated by weeks or months of being fine, herpesvirus reactivation is a strong possibility.
Uveitis: Inflammation Inside the Eye
Sometimes the problem isn’t on the eye’s surface but deeper inside. Uveitis is inflammation of the eye’s internal structures, including the iris and the tissue behind it. It causes squinting and tearing, but it can also produce changes you might not immediately connect to an eye problem: a shift in iris color, a pupil that looks smaller than the other eye’s, cloudiness, or even visible blood inside the eye.
What makes uveitis concerning is what can cause it. While trauma is one possibility, uveitis in cats is frequently linked to systemic infections like feline leukemia virus, feline immunodeficiency virus (FIV), feline infectious peritonitis (FIP), and toxoplasmosis. Cancer can also be a cause. A cat with uveitis often needs blood work and additional testing to identify the underlying condition driving the inflammation.
What to Look for at Home
Before you call your vet, take note of a few things that will help with diagnosis:
- Discharge type: Clear and watery suggests viral infection or mild irritation. Thick, colored, or crusty discharge points toward bacterial involvement.
- Cloudiness: A hazy or bluish film over the eye can indicate a corneal ulcer or internal inflammation.
- Third eyelid visibility: If the translucent inner membrane is partially covering the eye, your cat is actively trying to protect it.
- Pupil size: Compare both eyes. If the squinting eye’s pupil is noticeably smaller or larger, that suggests a deeper problem like uveitis or glaucoma.
- Duration: A single brief episode that resolves within an hour or two may have been a stray hair or dust particle. Squinting that persists beyond a few hours, or that comes and goes over days, needs professional evaluation.
Why You Shouldn’t Use Human Eye Drops
It’s tempting to reach for whatever eye drops you have in the medicine cabinet, but many human eye products are genuinely dangerous for cats. Anti-redness drops often contain decongestants from a drug class called imidazolines (the active ingredient in many “get the red out” products), which are highly toxic to cats. Even Neosporin-type antibiotic ointments, which seem harmless, have been linked to anaphylaxis in cats, a potentially fatal allergic reaction that can develop within four hours of application.
Saline rinse is the only thing that’s generally safe to use at home, and only to gently flush out visible debris. If the squinting continues after flushing, the problem is beyond what home care can address. Applying the wrong product to an ulcerated eye can make things dramatically worse, turning a treatable problem into a vision-threatening one.