Dilated pupils are pupils that are larger than normal. They are sometimes called dilated eyes .

The size of your pupils is controlled by tiny 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.

Normal pupil size generally ranges 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.

In one study of 500 Americans ages 18 to 34 years, average pupil sizes in three different lighting conditions were found to be:

• 3.35 mm in direct light

• 3.86 mm in normal room lighting

• 6.41 mm in near-total darkness

If your pupils are significantly larger than these averages, you have dilated pupils.

A dilated pupil can sometimes still react to light — that is, get smaller in bright light or when a light is shined at the eye. But typically, dilated eyes 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 , 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% of the population.

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The most common dilated pupil causes include:

Medications

The following prescription and non-prescription medicines can cause your pupils to dilate and affect their ability to react to light:

• Antihistamines

• Decongestants

• Tricyclic antidepressants

• Motion sickness medicines

• Anti-nausea medicines

• Anti-seizure drugs

• Medications for Parkinson's disease

• Botox and other medications containing botulinum toxin

• Atropine (used for myopia control and other medical purposes)

READ NEXT: Floppy Iris Syndrome

Eye injury

A serious, penetrating eye injury can damage your iris and cause the pupil 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

A head injury , stroke or brain tumor can affect how your pupils react to light and cause dilated pupils. One or both eyes may be affected.

This is why you see physicians checking an athlete's pupils with a penlight following head trauma sustained during sporting events, 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, the substances themselves do not cause your pupils to dilate.

A number of illegal drugs, however, do directly cause dilated pupils. This slows your eyes' ability to react to light.

These drugs include:

• Amphetamines

• Cocaine

• LSD

• MDMA (Ecstasy)

• Methamphetamine

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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 function without treatment.

Adie's pupil

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. Sometimes, the pupil does not constrict at all.

This 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.

Congenital aniridia

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 .

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Sexual attraction

It's true — researchers have found that pupil dilation appears to correspond to adult men and women's sexual interest in other adults. But there's a catch.

One recent 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.

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 your prescribing doctor 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 it comes on suddenly.

If you have dilated pupils or your pupils react slower than normal to changing light conditions, you will be more sensitive to sunlight. Eyeglasses that darken automatically in daylight or sunglasses with polarized lenses can make your eyes feel more comfortable outdoors.

Custom prosthetic contact lenses can also help reduce light sensitivity caused by dilated eyes. 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.

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