Dilated pupils in dogs can be completely normal or a sign of something that needs veterinary attention. In dim lighting or during excitement, a dog’s pupils naturally widen to let in more light or as part of an adrenaline response. But if your dog’s pupils stay large in bright light, look uneven, or come with other changes in behavior or appearance, something else is going on. The cause ranges from age-related muscle thinning to serious conditions like glaucoma or poisoning.
Normal Reasons for Dilated Pupils
A dog’s pupils adjust to light the same way yours do. In a dim room or outside at dusk, both pupils widen to gather more light. During play, stress, or a trip to the vet, adrenaline causes temporary dilation as part of the fight-or-flight response. This kind of dilation is symmetric (both eyes match) and resolves once the lighting changes or the excitement passes.
Some medications also cause temporary dilation. If your dog recently had a vet visit, eye drops used during the exam can keep the pupils wide for several hours afterward. This is expected and wears off on its own.
Poisoning and Toxic Exposures
Dilated or glassy-looking eyes are one of the hallmark signs of cannabis (THC) intoxication in dogs. With edibles, discarded joints, and THC-infused products increasingly common in households, accidental ingestion is one of the more frequent causes of sudden pupil dilation in otherwise healthy dogs. Other substances that cause dilation include chocolate, certain medications (especially antidepressants and antihistamines), and some plants.
If your dog’s pupils are suddenly wide and they seem wobbly, disoriented, unusually sleepy, or are vomiting, poisoning is high on the list. Think about what your dog could have gotten into in the last few hours. This warrants a call to your vet or an animal poison control hotline right away.
Glaucoma: A Painful Emergency
Glaucoma is one of the most serious causes of dilated pupils in dogs, and it requires emergency treatment. It happens when fluid inside the eye can’t drain properly, causing pressure to build. Normal eye pressure in dogs tops out around 20 to 28 mmHg. Pressures above 40 to 50 mmHg require immediate intervention to prevent permanent vision loss.
A dog with glaucoma typically has a pupil that looks dilated and doesn’t respond to light. The affected eye often appears red, cloudy, or has a bluish tint, and it may bulge noticeably compared to the other eye. Dogs with glaucoma show obvious signs of pain: pawing at the eye, rubbing their head against furniture, squinting, whining, or becoming withdrawn and uninterested in food. The pressure creates what amounts to an intense headache.
If you notice a bulging eye, sudden blindness, intense redness, or signs of severe pain alongside the dilated pupil, get to an emergency vet immediately. Glaucoma can cause irreversible damage to the retina within hours.
Sudden Vision Loss
A condition called Sudden Acquired Retinal Degeneration Syndrome (SARDS) causes blindness that develops over just a few days. The pupils become widely dilated and barely respond, or don’t respond at all, to light. What makes SARDS tricky is that the back of the eye looks relatively normal on initial examination, so the blindness can seem to come out of nowhere.
You might notice your dog bumping into furniture, hesitating at stairs, or startling easily. Progressive retinal atrophy, a slower inherited condition, causes similar pupil changes but develops gradually over months to years, often starting with difficulty seeing in low light before advancing to full vision loss. In either case, persistently dilated pupils paired with signs of vision trouble point toward a retinal problem.
Iris Atrophy in Older Dogs
In senior dogs, the muscles of the iris thin out over time, a process called iris atrophy. According to the American College of Veterinary Ophthalmologists, as this develops, the inner edge of the iris changes from a smooth, circular shape to one that looks rough or moth-eaten. The pupil gradually becomes more dilated and less able to constrict when exposed to light.
This is a common age-related change, not an emergency. It doesn’t cause pain, and most dogs adjust well. You may notice your older dog squinting more in bright sunlight since their pupils can no longer tighten down as effectively. It’s worth mentioning at your next vet visit, but it doesn’t require urgent care on its own.
Neurological Conditions
The pupil is controlled by nerves that connect the eye to the brain, so neurological problems can show up as dilation. Canine dysautonomia is a degenerative condition affecting the autonomic nervous system, the part of the nervous system that handles automatic functions like pupil size, digestion, and heart rate. Dogs with dysautonomia lose their pupillary light reflex, meaning the pupils stay fixed and wide, but they can still see. That combination of dilated pupils with intact vision is a distinctive clue.
Other neurological causes include head trauma, brain tumors, and inflammation of the brain or the nerves serving the eye. These typically come with additional symptoms like unsteadiness, head tilting, circling, or changes in consciousness.
One Pupil vs. Both Pupils
Pay attention to whether the dilation affects one eye or both. When both pupils are equally dilated, systemic causes like poisoning, fear, or low light are more likely. When only one pupil is dilated (a condition called anisocoria), the problem is more likely localized: glaucoma in that eye, an injury, nerve damage on one side, or inflammation within that specific eye.
Anisocoria is almost always worth a vet visit because it points to something asymmetric happening in the eye or the nerve pathways that control it.
How to Check Your Dog’s Pupils at Home
You can do a simple check by dimming the lights in a room so you can clearly see your dog’s pupils, then shining a flashlight (a bright one, not just a dim penlight) into one eye. The pupil in that eye should constrict quickly, and the opposite eye’s pupil should also constrict slightly. Pull the light away for a few seconds, then repeat on the other side.
What you’re looking for is symmetry. Both pupils should respond at roughly the same speed and shrink to a similar size. If one pupil stays wide while the other constricts, or if neither pupil responds at all, that’s meaningful information to share with your vet. A sluggish or absent response in bright light is not normal and suggests the eye or the nerve pathway has a problem.
Signs That Need Urgent Attention
Not every case of dilated pupils is an emergency, but certain combinations of symptoms should prompt an immediate vet visit:
- Eye bulging or size difference: one eye looks larger or is protruding from the socket
- Redness, cloudiness, or a blue tint in the eye alongside dilation
- Signs of pain: pawing at the eye, squinting, rubbing the head, whining, or reluctance to eat
- Sudden blindness: bumping into objects, hesitating to move, or startling at touch
- Neurological changes: wobbliness, disorientation, head tilt, or altered consciousness
- Suspected poisoning: dilation plus lethargy, vomiting, tremors, or unusual behavior after possible exposure to a toxic substance
If the dilation is in both eyes, your dog is acting normally, and the pupils still respond to light, it’s reasonable to monitor and bring it up at a routine appointment. But when dilation is persistent, one-sided, or paired with any of the symptoms above, sooner is always better.