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Why Are My Cat’s Eyes Dilated?

Cat with dilated eyes
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Your cat’s eyes are amazing. They can detect movement and see in the near-dark far better than we humans can. In fact, they’re perfectly adapted to hunting in low light levels. 

If you’ve been paying attention to your cat’s eyes, you might have seen them dilate every now and again. Read on to find out why your cat’s eyes are dilated and when it’s a cause for concern.

Cat Pupils: How Do They Work?

Your cat’s eyes have the same basic components that yours do – the visible areas of the eyeball are the sclera (the white bit), the iris (the colored area) and the pupil (the black center). 

Feline eyes have vertical slit-shaped pupils – this shape of pupil is common to predators that hunt in the low light levels of dusk and dawn, so it’s thought to be perfectly adapted to help see in low light.  

The pupil lets light into the back of the eye, where receptors ‘read’ the light and form nerve pulses. Your cat’s brain, when it receives these signals, turns these nerve pulses into a picture. The pupil can change in shape and size due to the muscles inside the eye. This allows more or less light to be let in, meaning the eye can adapt to different levels of light. 

In bright light, the pupil closes tight, preventing too much light getting in. Too much light can lead to temporary (or, in extreme cases, permanent) blindness and pain – like when you’re in a dark room and somebody turns a light on without warning. In low lights, the pupil opens wide (it dilates) letting as much light as possible in to help with vision. We call this wide-open pupil ‘mydriasis’.

Cat Eye Dilation: What Does it Look Like?

When your cat’s eyes are dilated, their pupils are larger than normal. At this size, the pupil is almost round, rather than slit-shaped. If you look closely, you’ll notice the ring of the iris (the colored portion of the eye) is much thinner than usual. 

You might notice the pupil being dilated temporarily while in low light, or adjusting to bright light, and this usually lasts only while they’re in low light, or for just a second or two as they transition into bright light. However, sometimes you’ll notice dilated pupils in cats even when they aren’t in low light. This can be a cause for concern, especially if the dilation is persistent.

Why Are My Cat’s Eyes Dilated?

Black cat with dilated eyes

Cat eyes can dilate for a number of reasons, ranging from normal to serious. The common causes of cat eye dilation are:

Response to Low Light

The normal response of a cat’s eyes to low light is to make the pupils big, therefore allowing plenty of light to see by. If you’re looking at a cat on a night-vision camera, or your cat wakes you up in the early hours, you might notice their pupils are really big. This is normal! This is a reflex action that cannot be controlled intentionally.

Excitement and Fear

Your cat’s eye muscles are also under the control of the sympathetic nervous system. This is the ‘fight or flight’ response. When your cat is stressed or excited – anything that might result in a release of adrenaline – this system dilates the eye, as if to prepare for fight or flight. This usually lasts for a short time, no more than a few minutes, and should fade as the cat calms down.


Related to the fight and flight response, pain can also trigger the sympathetic nervous system and cause dilated cat eyes. This usually only lasts for seconds (up to a minute) and then their pupils return to a normal size as their body adapts.


Catnip contains nepetalactone, which is well known for giving cats a ‘high’. While we aren’t sure exactly why cats respond to catnip this way, one of the symptoms of a cat on catnip is dilated eyes. They are usually back to themselves after around 10 minutes and won’t respond again for a couple of hours. Many cat toys contain catnip. If you’ve just given your cat a new toy and they’ve got dilated eyes, this is the likely cause!


Some toxins — including human drugs, illegal drugs, and some mushroom species — can cause your cat’s pupils to dilate. This is usually due to activation of the nervous system. 


Some medications can cause cat dilated eyes. One example is atropine, an eye drop that might be applied by your vet during investigations, or may be prescribed to help with an eye problem. Other medications that commonly cause eye dilation in cats include pain relief medications (especially opioids) and anti-anxiety drugs.


Glaucoma is a painful eye condition where the pressure in the eye is too high. It can cause many symptoms, one of which is cat dilated pupils. This might happen to one eye or both at the same time. 

High Blood Pressure

High blood pressure is a relatively common condition in cats, particularly senior and geriatric cats, especially if they have kidney disease or hyperthyroidism. High blood pressure causes damage to the eyes, and the eyes dilate in response. Sometimes this dilation from high blood pressure is treatable, but sometimes it isn’t. However, it’s still worth trying treatment as high blood pressure is damaging to many other organs, too.

Eye Dilation in Cats: Should You Worry?

If your cat’s eyes dilate but then return quickly to normal, you don’t usually need to worry. This is especially true if you have a good explanation for why it happened (such as a dark room, a loud noise, a sprinkle of catnip, or in the middle of a game of chase). 

If your cat’s eyes stay dilated, it’s more of a cause for concern and it’s a good idea to get an appointment scheduled with your veterinarian as soon as possible. 

As discussed above, high blood pressure (which often goes hand-in-hand with kidney disease, heart disease, or hyperthyroidism) can cause dilated pupils in cats. You might notice other signs of disease in these cats including changes in appetite or thirst, lethargy, or panting with the mouth open. If just a single pupil is dilated (termed ‘anisocoria’), this is also a concern, and again this should be investigated. 

Lastly, you should contact your vet if toxicity is a possibility – especially if your cat may have eaten any drugs, including recreational drugs. Your vet won’t judge or call law enforcement, they just want to treat your cat.

Diagnosing and Treating Abnormal Eye Dilation in Cats

Cat eye exam

Your veterinarian will start by taking a thorough history. Sometimes the cause of dilation will be known, such as if a toxin has been eaten (or breathed in). If that doesn’t identify the cause, your vet will examine your cat. 

Don’t be alarmed if your vet doesn’t immediately look in your cat’s eyes – there may be clues to what’s going on in their full clinical exam. Expect your vet to listen to your cat’s heart, feel your cat’s abdomen, and even check your cat’s gums. They will likely look in your cat’s eyes, and do some reflex tests with lights. If glaucoma is suspected, they may test your cat’s eye pressure.

Next up, blood tests and blood pressure measuring may identify the cause of the problem. In some cases, your vet’s examination will identify a neurological problem or an eye problem, in which case they may refer you to a specialist. This may also be necessary if your vet doesn’t have the specialized equipment necessary to make a full diagnosis.

Treating Eye Dilation in Cats

There is no specific treatment for eye dilation in cats. Treatment depends on the underlying cause. Your vet might recommend pain relief (eye drops or oral meds), medication to control the blood pressure (usually pills), a diet change (for kidney disease), or even an operation (in some cases of hyperthyroidism). If you have any questions, you can always ask your vet to explain what they’re treating, and why. 


There are lots of reasons a cat’s eyes dilate. If you notice your cat’s eyes are dilated and they aren’t returning to normal, or if one eye is dilated when the other is not, it’s time to contact a veterinarian for a check-up.