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Why Are My Dog’s Pupils So Big?

February 12, 2021

Have you ever noticed your dog’s pupil? Did you see anything strange about them? Have you ever questioned yourself why my dog’s pupils are so big?

Just like humans, dog’s pupils also get enlarged or contracted based on light. Generally, when your dog is playing in the sunlight, his or her pupils will contract, and similar when in the dark, they will enlarge.

But is that all? Or there’s more mystery to the dog’s pupil.

In this article, you will unravel some crucial details about the dog’s pupils and the possible issues that concern them.

Why Your Dogs Pupils Get Enlarged?

You might have wondered why your dog’s pupil get big sometimes. There are many reasons for it. Dog’s pupils are dark black circles at the center of the colored section of the dog’s eyes.

Interestingly, the dog’s pupil is not black, and it’s a matter of optical illusion. Dog’s pupil is holes that become black when light rays enter the pupil and get absorbed by the eye tissue. 

The primary function of a dog’s pupil is to manage the amount of light entering the eye. In dark surroundings, dog’s pupils get dilated (enlarge) to let more light into the eye, and similarly, when in the bright light environment, they will automatically constrict to restrict the amount of light into the eye.

The constricting of a dog’s pupil is called miosis, and dilating is known as mydriasis. The physiological phenomenon of dog’s pupil dilating and constricting is called pupillary response.

There are many reasons for the constriction and enlargement of the dog’s pupils. 

What is Pupil Dilation in Dogs?

Before delving into the possible causes of pupil dilation, it is crucial to understand what is pupil dilation. Pupil dilation is a slight change in the dog’s pupil.

During pupil dilation, the pupils look large, making the iris barely visible. It is essential to take a close look at your dog’s pupil to make sure that pupil dilation is not causing any health trouble.

The pupil will cover all the eye part. To learn more about the pupil dilation, you would need to get acquainted with its possible causes. Without further ado, take a look at the below reasons carefully to start examining your dog’s pupils.

What Causes Pupil Dilation in Dogs?

Below are the two primary reasons behind pupil dilation in dogs.

When it comes to pupil dilation in dogs, emotions play a significant role. It is one of the common reasons why your dog’s pupil dilates.

If you have been noticing your dog’s body language, you would note the change in their pupil size due to their emotional conditions.

While the dark surroundings majorly cause their pupil to dilate, there are some instances where the psychological state of the dog makes the pupil dilate.

When the sympathetic nervous system of your dog is triggered due to any emotion, be it fear, stress, excitement, etc., their pupil will dilate. It is important to notice such emotional instability that is combined with pupil dilation.

For instance, when you take your dog to the vet, there are instances when they pant heavily caused by stress, this fear or threat also results in pupil dilation.

When your dog is excited to do something such as to go on a walk or play with its favorite toy, that excitement also makes its pupil enlarge. So, all the dog’s emotions are the possible causes of pupil dilation. 

When you bring your dog out to play, that is possibly the best time of the day for your dog. Interacting with other canines and seeing the outer world is something they enjoy a lot.

Chasing the toy and running behind it is one of their favorite outdoor play games. When they are made to pursue something, they tend to activate their hunting mode. It turns on their predatory drive and triggers the adrenaline rush. 

During the adrenaline pump, more oxygen is flooded into the brain and transferred to the dog’s muscles to act quickly. Such quick response also reflects in their pupil when they want to see more of the object.

You must have noticed how excited your dog becomes when he or she is playing with you. And if you pay close attention, you will able to see pupil dilation in your dog. This is by far healthy for your dog, unless the adrenaline is exceptionally high for the dog.

While playing is not precisely hunting, the physiological response of the dog remains the same. When you throw the toy in the air, your dog takes it as a prey to catch that instantly turns on the hunting mode.

What Determines the Enlarged or Big Pupils In Dog?

Apart from the standard and fundamental reasons behind pupil dilation, there are several other unnatural reasons that you must be acquainted with to safeguard your dog’s health.

You must start examining your dog’s eye regularly to detect any unexplained changes in it. Familiarize yourself with the typical appearance of the pupil so that you can quickly identify the unnatural and deformity in your dog’s pupil.

If you notice any changes in your dog pupils which you cannot provide any reason, then you must visit a vet soon.

Take a look at the below causes of pupil dilation that you must be aware of.

Just like humans, dogs are also prone to birth deformity. They can also undergo deformities during birth. Their pupil size can be affected by such deformities. Some congenital disability directly affects the dog’s eyes.

For instance, some puppies are not born with a complete iris that result in a unilateral and bilateral change in their pupil size. If your dog were born with such deformities, the irregular pupil size would not be affecting your dog’s health.

So, you don’t have to worry about it much. Just learn to accept it and love your dog. 

Your dog’s pupils might be a possible cause of eye diseases. Three major eye diseases occur in dogs and cause a change in their pupil size.

1. Glaucoma

A common cause of blindness witnessed in older dogs, and glaucoma can happen to any dog’s breed. The increased pressure generally causes it in the eye that misplaces the aqueous humor. It is essential to diagnose glaucoma at the early stages so that the treatment can be deployed at the right time.

Glaucoma causes redness, inflammation, and itchiness in the eye and makes the dog uncomfortable. Dilation of the pupil is another significant sign of glaucoma.

If your dog’s pupil dilates even in the bright light, there are chances that your dog is suffering from glaucoma. It can be treated if diagnosed at the right time. Pay attention to your dog’s pupil so that you can detect such severe issues at the right time.

2. Retinal Degeneration

Less painful and irritating than glaucoma, progressive retinal degeneration is one of the concerning eye diseases in the dogs. Retinal degeneration can be a severe threat to your dog’s vision, as well.

Though it is not life-threatening, retinal degeneration can result in permanent loss of vision. It starts with blurred vision, and retina tends to decline the functions. Abnormal dilation of the pupil is also one of the prevalent symptoms of retinal degeneration. 

Owners often neglect this sign as they consider it as a common dilation, but it can be one of the visions threatening eye disease, i.e., retinal degeneration. It is crucial to remain attentive and vigilant about such changes in your dog’s pupils and get a necessary diagnosis done for the retinal degeneration.

You would not want your dog to lose the vision of some painful eye diseases. Hence, it is essential to keep a close watch on your dog’s vision.

3. Canine Anisocoria

An eye disorder that is occurred in both cats and dogs, anisocoria is a condition of unequal pupil size where one pupil looks smaller than the other. By detecting the issue at the earliest, there are medications and treatments for anisocoria.

Abnormal dilation of the pupil is another primary symptom of anisocoria apart from the small and large size of both pupils. Anisocoria can develop inflammation and redness in some dogs, while in others, it is merely painful. Every breed is affected differently by anisocoria.

It depends on the owner to detect the unequal size of the dog’s pupils and get a proper diagnosis from the doctor to ensure everything is fine with your dog’s vision. Ultrasound, CT scan, and MRI can be conducted to detect the possibility of anisocoria. It is advised to do the needful at the right time before things slip through your hand. 

Just like eye disorder, neurological disorders can result in abnormal pupil dilation. Eyes are directly connected to the central nervous system, and any issue in the nervous system will directly affect your dog’s pupils. Two major neurological problems can affect your dog, seizures, and cancer.

1. Seizures

Caused by excessive activity in the brain, seizures, or epilepsy is one of the common neurological disorders in dogs. The violent occurrences due to the neurons can result in muscle spasms. Such seizures in dogs often cause dilation of the pupil, followed by drooling that is exceptionally dreadful.

There are no possible symptoms of seizures, which means you cannot predict them earlier. All you can do is provide the necessary medication to bring your dog in a stable condition.

2. Cancer

Dogs are also prone to cancers. Just like in humans, there are different types of cancers in dogs, as well. Tumor development in the brain or inside the eye can result in several vision-related ailments, pupil dilation being the one. Suprasellar germ cell tumors formed near the pituitary glands are detrimental for your dog’s vision.

Dilated or unresponsive pupils are the common symptoms of this disorder. Cancer growth can also result in behavioral changes and declining eyesight. 

Your dog can be poisoned in many ways, and the toxicity from the poisonous substance can change your dog’s eye function. Such toxicity can also result in the dilation of pupils in dogs. There are plenty of toxins that can affect your dog’s pupil, including scorpion venom, marijuana, foxglove flower, etc. Keep your dog away from such poisonous substances and make sure they don’t come in contact with them. 

Physical injury is another significant cause of the dilation of pupils. If your dog has gone through some accident or trauma that might have damaged his/her optic nerve connected to the central nervous system, there are chances that your dog’s pupils might also get affected. Hence, it is vital to safeguard your dog from any physical injury that affects his/her pupil size or, more dangerously, the vision.

When Are Big Pupils a Sign of Disease or Illness?

Pupil dilation can be normal if it is caused due to natural reasons such as emotions, darker surroundings, etc. If your dog’s pupil is enlarging due to any unexplained reasons and causing discomfort to your dog, the chances can be that your dog might be ill or suffering from any disease.

The above mentioned were some of the possible illnesses and disorders that are witnessed in dogs and cause pupil dilation. You must remain attentive to such signs and give proper treatment to your dog at the right time. Early diagnosis can help in treating the disease in a better way.

Can Eye Medication Cause Pupil Dilation in Dogs?

Several eye diseases are found in dogs. For treating these eye diseases, dogs are given medications that can even impose side effects on them. Some medicines can affect adversely on the dog’s eye health. It is essential to make sure the drugs prescribed for your dog does not have side effects on pupils.

You cannot possibly detect the drugs that have side effects on your dog; you need to be aware of the possible symptoms after your dog consumes the medication. Once the medication start, you must pay close attention to the behavioral changes in your dog. Never neglect even the slightest changes in the physical and mental well-being of your dog. 

Are Some Dog Breeds More Likely to Have Smaller or Larger Pupils?

Pupil dilation and constriction can occur in any dog breed that means there are no specific breeds that are commonly affected. You need to identify whether your dog has been witnessing dilation and constriction of pupil without any natural cause.

Irrespective of the breed, pupil constriction and dilation can be caused to any dog. You don’t need to focus on specific breeds that have smaller or larger pupils. It’s the eye disorders that make develops irregularities in pupil size. Anisocoria is one such eye disorder that can influence the pupil size, and it can occur in any breed.

Do Mixed Dog Breeds Have Smaller or Larger Pupils?

As mentioned above, there are no particular breeds that are affected by the constriction or dilation of the pupils. Though there have some instances where some mixed breed dogs encountered smaller or larger pupils, there is nothing specific that those breeds were primarily affected. It can result from any congenital disorder or injury.


When you bring a pet dog into your life, their health and well being becomes one of your concerning matter. The way they look, behave, walk, or even sleep says a lot about them. If you love your dog, you probably cannot ignore the changes encountered in his or her body.

That goes for eyes too. If  you are thinking that why are my dog’s pupils so big, then you need to investigate.  Your dog’s health is your priority; anything that concerns your dog’s health should worry you as well.

Dog’s pupils are a sensitive body part. Though its dilation and constriction are considered normal, there can be some significant issues that might be causing it.

As a responsible owner, you must be able to detect any irregularities in your dog’s pupils’ size so that in case if there’s anything severe, proper medications can be given to your dog at the earliest.

Normal pupil dilation can be caused by anything, be it emotions or playing with full excitement. But, the unexplained dilation of the pupil can be a warning sign of possible diseases. Though it can be cured with medications and treatment, it is essential to diagnose it at the early stages to avoid any complexities.

Hence, you need to carefully watch the conditions of your dog’s pupils and check whether they are showing any changes in size. If those changes are natural, you don’t have to worry, in case, they are resulting from any unexplained reason, you need to contact your vet at the earliest and diagnose the condition of your dog.

The above mentioned were all the possible things you need to know about the pupil’s dilation in your dog. Research more about the dilation and constriction of dog’s pupils so that you can get a better insight regarding the matter. Make sure to remain vigilant about the changes in your dog’s pupil size, so that you can safeguard your dog’s health from severe diseases. 

Your dog relies on you for several matters, and health is one of the prominent issues. Don’t be negligent towards your dog’s health and keep a close eye on your dog’s changing behavior, both physically and mentally.