Updated on  February 20, 2024
4 min read

How Mydriasis Affects the Eyes

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Dilated pupils are when the black circles at the center of your eyes are bigger than normal. Muscles in the iris (colored part of your eye) control pupil size.

Pupils naturally expand or dilate in response to certain events, like dimming light. They usually constrict or shrink back to their normal size in bright light.

Mydriasis, also known as a fixed pupil, is when your pupils dilate without changes in light. They may stay dilated, even in bright light.

Close up of a dilated pupil during an eye examination

What Do Dilated Pupils Look Like?

When pupils dilate, they expand in size. Therefore, pupils look extra big when they’re dilated. The black pupil takes up more of the iris, so the eye may appear darker with less color.

Sometimes, one pupil is more dilated than the other. This common condition is called anisocoria. It affects about 20% of the population. 

With anisocoria, both pupils respond to light changes. However, one pupil is more than half a millimeter larger than the other.


What Do Normal Pupils Look Like?

On average, a normal pupil in adults is:

  • About two to four millimeters in diameter in bright lights
  • Four to eight millimeters in diameter in the dark (dilated)
  • Generally, both pupils are about the same size

Normal Causes of Dilated Pupils

Pupils dilate for many reasons, including:

  • To see better in low light
  • When the eye focuses on a distant object
  • Dilating eye drops used in eye health exams
  • Sexual attraction (due to an increase in the hormone oxytocin)
  • Reactions to certain medications

Pupils are supposed to dilate; it’s a natural response to low-light situations. Pupils may dilate in response to various stimuli under completely normal circumstances.

However, if the pupils remain dilated or are unequal in size, seek care from your ophthalmologist. You could have a medical emergency on your hands. 

Mydriasis can be a sign of serious conditions that affect the brain, such as:

  • Stroke
  • Tumor
  • Head trauma

Medical Causes of Dilated Pupils

Pupil dilation often occurs on its own, with no other causes. In other cases, it can result from medication or an underlying condition, especially if it’s prolonged or unusual.

Potential causes of mydriasis include:


Various prescription and over-the-counter medications can cause pupil dilation, including:

  • Atropine
  • Botulinum toxin
  • Antihistamines like diphenhydramine
  • Decongestants like pseudoephedrine
  • Tricyclic antidepressants 
  • Anti-seizure medications
  • Anti-nausea and motion sickness medications such as Dramamine
  • Parkinson’s disease medications
  • Stimulant medications that treat ADHD

Recreational Drugs

Recreational drugs that can cause mydriasis include:

  • LSD
  • Cocaine
  • MDMA (Ecstasy)
  • Methamphetamines
  • Psilocybin mushrooms

Pupil dilation can also be a sign of withdrawal from these drugs.

Benign Episodic Unilateral Mydriasis (BEUM)

BEUM is a temporary condition that causes one pupil to dilate. The larger pupil usually returns to normal size in a few hours or days. Other symptoms include:

BEUM itself is not dangerous. However, you should undergo a comprehensive eye exam to ensure your eyes and nervous system are healthy.

Traumatic Brain Injury

A brain injury can increase your intracranial pressure and damage the iris. This often causes one eye to dilate, commonly called a “blown pupil.” Common causes include:

  • Head injury
  • Stroke
  • Tumor

Eye Injury or Surgery

An eye injury that damages the muscles or nerves in the iris can cause pupil dilation. Common examples of eye injuries include:

  • Getting punched in the eye
  • Getting hit by a flying ball
  • Chemical splashes

In addition to trauma, eye injuries can result from certain surgical procedures, such as cataract removal.

Microvascular Cranial Nerve Palsy (MCNP)

MCNP happens when blood flow to the nerves serving your eye is blocked. This can change pupil size and impair vision.

Adie’s Syndrome

Also known as Adie’s pupil, this rare neurological condition causes abnormal dilation in one or both eyes.

Cranial Nerve Neuropathy

Cranial nerve neuropathy is the gradual deterioration of nerves that connect to your eye. Your oculomotor nerve controls pupil constriction. So, any damage to it can cause mydriasis.

This can occur in one or both eyes. It is often accompanied by impaired vision.

Treatment for Abnormal Pupil Changes

Treatment for dilated pupils depends on the underlying cause. Seeing a doctor about abnormal pupil changes is important because they can affect your vision. 

Your doctor may recommend one of the following treatments:

Photochromic Lenses

Photochromic (transition) lenses automatically darken outside in daylight and lighten up inside in darker conditions. These can help reduce sensitivity to outdoor lighting.

Polarized Sunglasses

Polarized lenses protect your eyes from intense light conditions. They reduce glares from surfaces like water, snow, asphalt, and sheet metal by filtering out specific wavelengths.

Prosthetic Contact Lenses

Custom contact lenses can reduce light sensitivity due to eye dilation. They can also make it look like both pupils are the same size.


If you have mydriasis caused by an injury, you may require surgery to repair any damage.

Abstinence or Rehabilitation

If your dilated pupils are a side effect of substance use, you may consider rehabilitation.


  • Dilated pupils are when the dark centers of your eyes are larger than normal
  • Pupils dilate naturally in response to changing light
  • Mydriasis is when pupils stay dilated, even in bright light
  • Many things can cause mydriasis, including dilating eye drops, medication side effects, and traumatic injuries
  • Abnormal eye dilation can be a sign of a medical condition that needs treatment, such as a stroke
Updated on  February 20, 2024
8 sources cited
Updated on  February 20, 2024
  1. What Kind of Drugs Cause Dilated Pupils?” Lakeside Academy, n.d.
  2. Adie Pupil.” American Academy of Ophthalmology, 2022.
  3. Anisocoria and Horner’s Syndrome.” American Association for Pediatric Ophthalmology and Strabismus, 2021.
  4. Dilated Pupil.” UCLA Health, n.d.
  5. Lick, David J, et al. “The Pupils Are the Windows to Sexuality: Pupil Dilation as a Visual Cue to Others’ Sexual Interest.” Evolution and Human Behavior, 2016.
  6. Ou, Yvonne. “The Dilated Eye Exam: Why It’s So Important.” BrightFocus Foundation, 2021.
  7. Polarized Lenses and How They Work.” Dr. Floyd Smith, 2013.
  8. Transition Sunglasses Lenses.” American Academy of Ophthalmology, 2021.
The information provided on VisionCenter.org should not be used in place of actual information provided by a doctor or a specialist.