Why are my cat's eyes dilated?

Updated 29/09/2023

You might not know it, but there’s more to your feline friend’s resting cat face than meets the eye. While their facial expressions might not give us much of an insight into how they’re feeling, their eyes definitely do (some of) the talking. Along with an understanding of cat body language and behavior, finding out the answer to “why are my cat’s eyes dilated?” is important for cat parents to know - and can be a key step in discovering how they’re feeling and what they’re trying to express. 

What does cat eye dilation look like?

First of all, let’s do a little revision on the anatomy of the eye. Irises and pupils are the most “eye-catching” features of a cat’s eyes. The iris is the colored part of the eye and comes in various shades, often determined by the cat’s breed. The pupil is the black, slit-shaped area that sits inside the iris. When dilated, the pupil becomes large and round, taking up the majority of the eye so only a small portion of the iris is visible. This gives the cat the appearance of bigger eyes, but it is in fact just the pupils that have dilated and appear larger.

Why are my cat’s eyes dilated?

Cat’s eyes become temporarily dilated for a variety of reasons, from changes in light levels to a sudden rush of adrenaline (fight/flight response).

Low light conditions

Why do cats’ eyes dilate? Low light conditions are the most common reason. When in a dimly lit environment, the normally slit-shaped pupils of the cat’s eyes grow large and circular in shape, allowing them to absorb as much light as possible and making it easier for them to see in the dark. Feline friends use the dimlight hours at dawn and dusk to hunt, and their pupils instinctively adapt to the low light so they can see their prey better. On the flip side, bright light causes the pupils to constrict to protect the cat’s eyes from absorbing too much light.

Excitement or fear

A cat’s pupils can also dilate when they’re in fight or flight mode, which causes a rush of adrenaline to the kitty’s eye muscles. This can make it hard to know whether your feline friend’s feeling excited or stressed out by a situation, so it’s important to pay attention to other body language cues, particularly those expressed through their tails.

Their pupils will also be dilated when they’re excited during a play session, as they pounce and dive on their toys!

Pain

Just like with fear and excitement, the fight or flight response can be triggered by pain. A sudden shock or momentary twinge may be why your cat’s eyes are dilated. 

Catnip

Catnip contains nepetalactone, a chemical compound that gives kitties a euphoric feeling of being “high”. Chewing or sniffing catnip might cause your fur baby’s eyes to dilate, and they’ll often run around with excitement for ten minutes until the catnip wears off.

Why are my cat’s eyes always dilated?

Wondering 'why are my cat’s eyes dilated all the time'? Temporary dilation in cats is usually nothing to worry about, but if your fur baby’s eyes are constantly dilated this could be cause for concern.

Medication side effects

Dilated pupils in a cat can be a side effect of pain relief medication, anti-anxiety drugs, and certain eye drops. 

Substances or toxins

Exposure or ingestion of toxins such as human drugs and some species of mushrooms may be why your cat has big pupils. 

Neurological disorders

Continuous cat eye dilation may be a sign of a neurological condition affecting the brain. This may be a degenerative condition or may be caused by trauma e.g. a road traffic accident. It’s likely that your cat will show other signs such as a change in behavior, abnormal walking (ataxia) along with a change in their appetite or drinking. Cats that have been hit by a car are likely to have other injuries such as broken bones or lung injuries.

High blood pressure

Want to know “why are my old cat’s eyes always dilated”? High blood pressure in cats, otherwise known as feline hypertension, can cause the pupils to be constantly dilated and fail to constrict in bright light. It can cause blindness if it’s not treated.The condition often occurs along with kidney disease or hyperthyroidism, both of which are common in elderly cats.

Medical conditions 

Other medical conditions that can cause a cat’s pupils to dilate include glaucoma, seizures, and cancer. 

So what does it mean when cats’ eyes are dilated? Whether it’s a response to low light levels or a sign of health problems, being aware of what causes dilated pupils in cats is an important responsibility for pet parents and cat sitters. Remember to take feline friends to the vet for examination if your cat’s pupils stay dilated for longer than a few minutes, especially if there’s no plausible explanation for it. Your vet will be able to tell you whether a medical condition or disorder is causing your cat’s pupils to dilate and will check for any signs your fur baby has ingested something toxic. 

Meet our veterinary expert, Aisling

This article has been checked by veterinarian Aisling O’Keeffe MVB CertSAM ISFMCertAdvFB. Aisling qualified as a vet 7 years ago from University College Dublin. She has worked in a mixture of UK small animal hospitals along with Irish practices. She worked for 3 years in a feline-only hospital where she further developed her feline medicine and surgery skills. She currently lives and works in a small animal hospital in Cork, Ireland.