Dilated Pupils: Causes, Symptoms, Treatments

Written By: Susannah Lockett

The eye pupil is the black center portion in the middle of the iris or the colored part of the eye. The pupil’s purpose is to regulate the amount of light entering the eye by dilating, enlarging, or constricting.

It’s not surprising to learn that the pupil changes size throughout the day in response to available light conditions. The pupils of a healthy eye will always dilate in lower light conditions.

You can see this happening in the eyes of others around you. If you have pet cats, you will notice their pupils, although slit and not round, become enormous in the darkness.

It’s just nature’s way of allowing as much light as possible to reach the eye’s retina. The retina is the light-sensitive membrane at the back of the eye that forms images and transmits them to the brain.

Normal Pupil Size

Tiny muscles located in the iris control the size of the pupils. Although pupil size tends to diminish with age, there is still a fairly stable range of normal pupil size. In people 18 to 34, this range is typically about 3.35mm in direct bright light, 3.86mm in normal room lighting, and 6.41mm in darkness.

A mm is a measurement equal to one-thousandth of a meter, approximately 0.039 inches.

If your pupils fall out of range of these figures, you probably have dilated pupils and should see an eye care professional as soon as possible.

There is an eye condition called anisocoria, which causes the pupils to be of different sizes. Anisocoria is not the same as dilated pupils.

It’s typically benign and affects about 20% of the population, but only an eye care professional can diagnose and differentiate anisocoria from dilated pupils.

No one can see in total darkness. For sight to occur, there must be some source of light. While it’s true that many animals, such as cats, have much better low-light vision than humans, there must still be some light for these animals to see.

The pupils respond to light shining directly in them by constricting. However, in people with dilated eye conditions, this normal response is usually compromised.

A dilated pupil failing to respond to light is typically the sign of a severe brain injury. Doctors call a pupil in this condition “fixed and dilated.” It may also be called a “blown pupil.”

Causes of Dilated Eyes

There are many possible causes of dilated eyes:


Many medications and illicit street drugs affect the eyes and can cause dilated and constricted pupils. We see this effect in two drug classes: stimulants and opioids.

Stimulants, such as amphetamines and cocaine, tend to cause the eye to dilate, while opioids cause them to constrict. This latter effect is sometimes called “pinpoint pupils.”

LSD and MDMA or ecstasy are two other illicit drugs capable of causing a temporary dilated eye condition. While alcohol and marijuana don’t directly cause the pupils to dilate, they can still reduce the eyes’ ability to constrict the pupils to normal size after sudden exposure to bright light. This may be an indirect cause of dilated pupils that can last two hours or more while under the influence of these drugs.

Other medications possibly causing the pupil to dilate include:

Anti-seizure drugs
Medications for Parkinson’s disease
Antidepressants called TCAs or tricyclic antidepressants
Drugs for nausea and motion sickness

Atropine is a medication derived from the belladonna plant used to dilate the eyes for examination and the control of myopia in children. It’s typically used in the form of drops, but there are many systemic medications containing atropine as well. All medications containing atropine or similar drugs may cause the pupils to dilate.


Belladonna means “beautiful lady” in Italian. As amazing as it seems today, Italian women in the 17th century would use belladonna plant extracts to enlarge their pupils. This was considered to be highly attractive at the time. This was also dangerous because belladonna derivatives like atropine are harmful when not used under medical supervision, especially in high doses.

Eye Injury

A serious eye injury penetrating the eye may damage muscles in the iris and cause a dilated pupil. The same is also possible during some types of eye surgery.

Brain Injury

A head injury, stroke, or tumor in the brain may affect how the eyes react to light. This is why you will see doctors using a penlight to shine light into the eye. If the pupils don’t react normally, it’s a sign of a possible brain problem or stroke.


Benign Episodic Unilateral Mydriasis

This benign condition, most commonly affecting young females prone to migraines, causes one pupil to dilate more than the other, often accompanied by eye pain, blurry vision, and headaches. Those affected may experience these episodes two to three times a month, each lasting up to 12 hours. The condition is harmless and requires no treatment.

Adie’s Tonic Pupil

This rare neurological disease causes one eye to dilate. The dilated pupil is typically slow to react to light and may fail to react at all. There may also be poor reflexes present.

No one knows what causes Adie’s tonic pupil, but possible causes include trauma, infection, surgery, or insufficient blood circulation. The condition is incurable.

Congenital Aniridia

Congenital means “present at birth.” People with congenital aniridia are born with defective or absent irises, which means the muscles normally controlling the pupil aren’t present.

It usually affects both eyes and may be present along with glaucoma, congenital cataracts, nystagmus, and incomplete retina and optic nerve development. Vision is severely compromised, with vastly reduced visual acuity and hypersensitivity to light.

Sexual Attraction

The pupils of both sexes may dilate in the presence of a desirable male or female.

Men’s pupils tend to dilate in the presence of an attractive female, but women’s pupils may dilate when close to attractive people of either sex, regardless of sexual orientation.

Treatment for Dilated Pupils

An undiagnosed enlarged pupil condition always requires a visit to an eye care professional. The cause may be serious and must be evaluated immediately.

If you notice your pupils are dilated, one is larger than the other, or you experience sudden dizziness, confusion, nausea, or changes in your balance or gait, seek medical help immediately. This is especially true if you also had some sort of head injury.

Although certain medications will produce dilated pupils as a known side effect, this should still be reported to your doctor.

Treatment for dilated eyes will depend on the cause. Custom prosthetic contact lenses for aniridia decrease the eye’s sensitivity to light and improve its appearance by making it look like normal pupils are present.

Special eyeglasses with polarized lenses may help others with dilated eyes feel more comfortable outdoors.

Find an Eye Doctor Near You

If you’re concerned about dilated pupils or any other eye problem, or you just need a regular eye exam or new glasses or contact lenses, our list of top-rated eye doctors will help you find a great eye care professional near you.

Even if you’re not having problems with your eyes, an annual eye exam is necessary to ensure your eyes are healthy. Some serious eye conditions may not show symptoms until they are advanced. Protect your eyes and your health by visiting an eye doctor regularly.

Published on: August 5, 2022
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