Dilated Pupils: Causes and Concerns
What are dilated pupils?
The size of your pupils is controlled by muscles in the colored part of your eye (iris) and the amount of light reaching your eyes. In bright light, your pupils constrict (get smaller) to prevent too much light from entering your eyes. In dim lighting, your pupils dilate (get larger) to allow more light in.
Generally, normal pupils range in size from 2.0 to 4.0 millimeters (mm) in bright light, and 4.0 to 8.0 mm in the dark. To some degree, pupil size tends to get smaller with age.
A dilated pupil sometimes can still react to light — that is, get smaller in bright light or when a light is shined at the eye. But typically, dilated pupils don’t respond normally to light.
A large pupil that is completely unresponsive to light is called a "fixed" dilated pupil.
Dilated pupils aren't the same as anisocoria, which is a common condition where both pupils react normally to light but differ in size by about a half millimeter or more. Anisocoria is benign and affects about 20 percent of the population.
What causes dilated pupils?
There's a wide variety of things that can cause dilated pupils. Here are some of the most common ones:
The following prescription and non-prescription medicines can cause your pupils to dilate and affect their ability to react to light:
Motion sickness medicines
Medications for Parkinson's disease
Botox and other medications containing botulinum toxin
Atropine (used for myopia control and other medical purposes)
A serious, penetrating eye injury can damage your iris and cause the pupil of that eye to become dilated and irregular in shape. Sometimes, this sort of injury can occur during an eye surgery, such as a complicated cataract surgery or a corneal transplant.
Brain injury or disease
This is why you see physicians checking an athlete's pupils with a penlight following head trauma sustained during sports competitions, or when a patient arrives at a hospital emergency department with other possible stroke symptoms.
Recreational drug use
Research has shown that alcohol and marijuana — separately or in combination — can reduce your eyes' ability to recover from exposure to a bright light source (such as oncoming headlights at night) and adapt to changing light conditions. This effect can last two hours or longer after drug ingestion.
However, these substances do not cause your pupils to dilate.
A number of illegal drugs used for recreational purposes, however, cause dilated pupils and slow your eyes' ability to react to light. These include:
Benign episodic unilateral mydriasis
This is an unusual but harmless condition where a person experiences sporadic episodes of one pupil suddenly becoming dilated, often accompanied by blurry vision, headache and eye pain.
Young women who are prone to migraine appear to have the highest risk of benign episodic unilateral mydriasis. In one study, the median duration of the episodes was 12 hours (some lasted much longer) and the median frequency was two to three episodes per month. The condition resolves and the pupil returns to normal size and functioning without treatment.
Also called Adie's tonic pupil or tonic pupil, this is a rare neurological disorder where one pupil is larger than normal and is slow to react to light or does not constrict at all. The pupil abnormality may be accompanied by poor or absent tendon reflexes. (When this occurs, the condition is called Adie's syndrome.)
Generally, the cause of Adie's tonic pupil is unknown; but in some cases, it may be associated with trauma, surgery, poor blood circulation or infection. There's no cure for Adie's pupil or Adie's syndrome.
This is a rare condition where a person is born with a partially or completely absent iris, resulting in a very large pupil.
Aniridia usually affects both eyes and is accompanied by other serious eye problems such as congenital cataracts, glaucoma, incomplete development of the retina and optic nerve, nystagmus, and decreased visual acuity.
Because there is little or no iris to regulate the amount of light entering the eye, people with aniridia are very sensitive to light.
Researchers have found pupil dilation appears to correspond directly to adult men and women's sexual interest in other adults.
One study showed that the pupils of male subjects dilated when they viewed images of women they found sexually attractive, whereas the pupils of female subjects typically dilated in response to images of attractive men and women alike.
The study authors concluded the reason for this is unclear and that further research is warranted.
What to do for dilated pupils
If you or someone else notices you have dilated pupils or one of your pupils looks larger than the other after head trauma, seek medical attention immediately.
The same is true if you experience sudden dizziness, headache, confusion, balance problems or other symptoms of a possible stroke.
If you notice dilated pupils after you've started taking one of the medications noted above, call the prescribing physician for advice.
In cases other than those described above, call your eye doctor immediately for advice if you notice your pupils are dilated — especially if the onset is sudden.
[If you don’t currently have an eye doctor near you, you can find one here.]
If you have dilated pupils or your pupils react slower than normal to changing light conditions, you will be more sensitive to sunlight. Consider purchasing eyeglasses with photochromic lenses that darken automatically outdoors in daylight for greater comfort. Or purchase sunglasses with polarized lenses for optimum comfort and glare reduction in bright sunlight.
Another option to reduce photophobia caused by dilated pupils and to improve the cosmetic appearance of the eyes is to be fitted with custom prosthetic contact lenses. These lenses give the appearance of having equal pupils of normal size. Prosthetic contacts are especially beneficial for cases of aniridia and large, irregular pupils caused by trauma.
Page published in December 2019
Page updated in April 2020