Pupil Size and Your Health

The size of your pupil can tell your healthcare provider quite a bit about your health. It's an important key to unlocking possible medical conditions you might not otherwise know about.

There are many parts of the eye, and the pupil is among the most important. It controls the amount of light that enters your eye. It also continually changes size.

Your pupil naturally widens and narrows based on the brightness of the light around you. It also changes size depending on whether you are looking at near or far objects.

This article discusses how the size of your pupils can give healthcare providers clues about your health and help them diagnose medical conditions.

pupils constantly change their size

What Is the Pupil?

The pupil is the round, black circle in the center of the iris. Your iris is the colored part of your eye. The pupil is actually a hole through which light passes to the retina, the light-sensitive layer in the back part of the eye.

Your pupils are similar to a camera aperture. They widen or narrow to let more or less light in. Pupils can expand to become larger (dilate) or contract to become smaller (constrict). 

Your iris contains muscles that respond to outside stimuli to control the amount of light that reaches your retina. In bright light, the pupil constricts to reduce the amount of light entering the eye. In dark or dim light, the pupil dilates to allow more light into the eye so you can see better.

Your pupils also constrict slightly to look at close objects and dilate slightly to look far away.  

Normal pupil size ranges between 1/16 to 5/16 of an inch (2.0 to 8.0 millimeters), depending on the lighting. The younger you are, the larger your pupils tend to be in normal light.

Testing Pupil Size

When your healthcare provider examines your pupils, they will first look for anisocoria—a condition in which the pupils aren't the same size. Twenty percent of the general population has slight anisocoria that does not signal anything abnormal. But in some cases, unequal pupil sizes can be a symptom of a disease.

Your healthcare provider will also look at the size and shape of your pupils in bright and dim light. Healthcare providers will note the quality and speed that your pupils respond to bright and dim light as well. They may also test your pupils' response to objects that are nearby, such as small print. Any differences between your pupils are also noted.

The optic nerve and oculomotor nerves both control pupil size. These nerves receive some of their messages from the autonomic nervous system. This part of the nervous system controls all of your vital functions, many of which are automatic.

A disruption of the autonomic nervous system might cause changes in how your pupils react to stimuli. That's why the size of your pupils can indicate health problems completely unrelated to your eyes.

Associated Conditions

Pupil size abnormalities can sometimes mean you have a disease. This is just a small sample of medical conditions you could have. That's because there are also other conditions that can cause irregular pupil function.

Conditions include:

  • Brain aneurysm: An aneurysm that pushes on nerve pathways in the brain can cause a dilated pupil, as well as other symptoms.
  • Lung cancer: Lung cancer that affects the top part of the lung can spread and impact the nerves that control the pupil.
  • Brain tumor: A tumor or mass close to the origin of the pupil's nerves can cause problems in the pupil's function.
  • Multiple sclerosis: Multiple sclerosis can cause optic nerve damage, which leads to an abnormal response of the pupils known as afferent pupillary defect (APD).
  • Head trauma: Head injury or concussion can cause unequal pupils.
  • Cluster headaches: Cluster headaches can cause a constricted pupil on one side.
  • Stroke: A stroke can sometimes cause changes in the size of a pupil.
  • Syphilis: Syphilis can cause an Argyll-Robertson pupil. These are small, unequal, misshapen pupils that constrict with near focusing but do not react normally to light.

In addition, recreational drugs and alcohol can cause the pupils to dilate or constrict abnormally. This is the reason why healthcare providers check your pupils when there is a concern about intoxication or overdose.

Some prescription and over-the-counter medications, including antihistamines and a few medications used to treat glaucoma, can sometimes dilate your pupils as well.

How Pupil Size Affects LASIK Surgery

It is possible that the size of your pupils can prevent you from having LASIK eye surgery to correct your vision. People with very large pupils are generally not good candidates for LASIK and other refractive procedures.

Eye specialists may use a device called an infrared pupillometer to measure the size of the pupils. The device consists of a large camera that shines infrared light (an invisible type of light) on the eye and senses the reflected light.

Having naturally large pupils or pupils that dilate heavily in dim light may increase the chances of glare and halos following LASIK. This would interfere with the clear vision you are hoping for from the surgery. For this reason, measuring pupil size is an important step in deciding if LASIK is right for you.


The size of your pupils can give your healthcare provider clues about your health. Pupil size constantly changes according to the amount of light entering it. It also changes depending on whether the objects you are looking at are near or far away.

When your healthcare provider notices your pupils aren't acting normally, it tells them that you may have a disease or medical condition. It can also tell healthcare provider if you are under the influence of recreational drugs or alcohol.

If your healthcare provider notices that you have naturally large pupils, you probably are not a candidate for LASIK surgery. That's because people with naturally large pupils may experience glares and halos following a LASIK procedure.

Frequently Asked Questions

  • What does pupil size indicate?

    Pupils naturally constrict and dilate to control the amount of light that gets in. This helps you focus your eyes on things both near and far. 

    Abnormal changes in pupil size can indicate a person has been drinking or is on drugs. Certain health conditions, like a brain aneurysm or head trauma, can also cause abnormal pupil size changes. 

  • Can a person have naturally large pupils?

    Yes. It is rare, but some people can have naturally larger pupils. The standard pupil size is 2 mm to 8 mm. People with naturally large pupils are prone to light sensitivity. The larger pupil allows more light in the eye, which can be uncomfortable but not harmful.

  • What do uneven pupils mean?

    Pupils that are not the same size are known as anisocoria. This can occur naturally or be a sign that something is wrong. Roughly one in five people have naturally occurring anisocoria.
    Conditions that can cause pupils to be uneven include a brain aneurysm, cluster headache, or stroke. 

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading

By Troy Bedinghaus, OD
Troy L. Bedinghaus, OD, board-certified optometric physician, owns Lakewood Family Eye Care in Florida. He is an active member of the American Optometric Association.