What Is Mydriasis?

Mydriasis is when pupils dilate, or widen, unnaturally—not in response to being in an environment with less light.

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A dilated pupil

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Pupils let light into your eyes and change size depending on how dark or light your surroundings are. In bright spaces, pupils are smaller since less light needs to enter your eyes. As you enter a darker environment, your pupils naturally widen (or dilate) to let in more light. But sometimes pupils dilate even when the light in your environment hasn’t changed. This occurrence is known as mydriasis.

Mydriasis may happen for various reasons, including in reaction to a medication you’ve taken or as a result of an eye injury, neurological condition, or traumatic brain injury. The cause of your mydriasis can dictate your treatment plan.


Your pupils are the black circle in the center of your iris (the colored part of your eye). They can be anywhere from 2-4mm in diameter, depending on how light or dark your surroundings are. Pupils dilate when exposed to darkness to let in more light so that you can see. Once exposed to light again, your pupils will go back to their smaller size.

 However, when mydriasis occurs, several out-of-the-ordinary things may happen:

  • Your pupils may dilate even without being exposed to darkness.
  • Your pupils may stay dilated even when exposed to light.
  • One of your pupils may be a different size than the other.

 Depending on the cause, you may also experience symptoms such as:


There are a number of reasons why you may experience mydriasis.

Dilated Pupils During Eye Exam

The most common cause of mydriasis is when a healthcare provider dilates your pupils during an eye exam. The provider will put eye drops in your eyes to cause the pupils to dilate. Dilating your pupils allows your provider to see into your eyes better and examine you for conditions like glaucoma and diabetic retinopathy (a diabetes complication that can lead to vision loss).

If your pupils are dilated during an eye appointment, it’s recommended that someone else drives you home, as eye dilation can cause blurry vision. This type of dilation goes away after a few hours and is not harmful.


Medications may cause mydriasis. Many of these medications are called anticholinergics, which treat a wide range of conditions by hampering parts of the nervous system.

Common medications that can cause mydriasis include certain types of:

  • Antihistamines (allergy drugs)
  • Tricyclic antidepressants 
  • Bronchodilators
  • Parkinson disease drugs
  • Bladder relaxants
  • Antipsychotics

Usually, the dilation ends once the medication leaves your system. Depending on the medication, you may also experience other side effects, such as blurred vision and dizziness.

Drugs with botulinum toxin A, such as Botox (onabotulinumtoxinA), might also cause mydriasis.


Certain drugs can cause mydriasis. Substances that may cause your pupils to dilate include:

  • Marijuana
  • Cocaine
  • Methamphetamines 
  • LSD (lysergic acid diethylamide)
  • Bath salts (synthetic cathinones)

The abuse or misuse of opioids, prescription stimulants like those for attention-deficit hyperactivity disorder, and nasal decongestants can also cause pupil dilation.

Injuries to the Eye

Sometimes an injury to your eye can cause your pupils to unnaturally dilate. This can happen from an accident, such as a ball or other flying object hitting your eye. Eye injuries may also happen as a result of toxic substances making contact with your eyes. In addition to pupil dilation, you can experience changes in vision or even blindness. It’s important to seek emergency medical care if you sustain a serious eye injury.

Adie’s Pupil

Adie’s pupil is a rare neurological disorder that causes your pupils to respond abnormally. Often, one pupil will be larger than the other and won’t shrink back to normal size when exposed to light. Other vision changes may be present, such as blurry vision and sensitivity to light. It may be challenging to read a book. Experts are unsure what causes Adie’s pupil, but it may be linked to bacterial infections, viral infections, or autoimmune disorders.

Microvascular Cranial Nerve Palsy

Microvascular cranial nerve palsy is a condition that occurs when you experience blocked blood flow to your cranial nerves. Besides dilated pupils, other symptoms of microvascular cranial nerve palsy include double vision. High blood pressure and diabetes are thought to cause microvascular cranial nerve palsy. Symptoms usually resolve in six to 12 weeks. A third nerve palsy resulting from aneurysm, although rare, is an emergent condition and should be evaluated on an urgent basis.

Traumatic Brain Injury

Traumatic brain injuries occur after serious injuries to the head. A person with a traumatic brain injury may experience loss of consciousness, memory issues, neurological symptoms, and difficulty concentrating. Mydriasis is also a possible symptom. Sometimes only one pupil will be dilated, suggesting a substantial, one-sided injury may be present.


Migraine doesn’t always cause pupil dilation, but some types of migraine attacks may. Migraine with aura is a severe migraine headache that was preceded by visual and neurological symptoms like flashes of light and blind spots. These attacks might cause your pupils to dilate.


If you notice that your pupils are dilated without a known cause, you should seek medical attention. Your healthcare provider will ask you questions about your symptoms and may discuss medications or drugs you’ve taken. They will examine you and may take blood samples or perform other diagnostic exams.

Your provider will likely do a thorough eye exam and take note of how your pupils respond to light and darkness and whether one pupil is larger than the other. (Unequal pupil size is known as anisocoria, and there can be various reasons for the difference in size, many of which can be serious.) The provider will also look for any other abnormalities about your eyes or your vision.


Treatment for mydriasis will depend on the cause. The goal of treatment will be to return your pupils to their normal size and to treat any underlying conditions that may be causing the abnormality.

Treatment may include the recommendation that you change medications. Your physician may also suggest that you wear special sunglasses that are light-sensitive or contact lenses that can help correct mydriasis.

If your pupils were dilated as part of a routine eye exam, no treatment is needed; your pupils will return to their normal size in a few hours.

When to See a Healthcare Provider

If your pupils become dilated for no obvious reason, you should contact your healthcare provider. It’s especially important that you contact your provider if:

  • The dilation is just in one eye
  • You’ve recently experienced an eye injury or blow to the head
  • You have other concerning symptoms such as blurry vision, disorientation, or dizziness, double vision, or a drooping lid.

Certain conditions that cause mydriasis are serious, like a traumatic brain injury, and should not be ignored.

A Quick Review

Usually, your pupils dilate when the light in your environment becomes dimmer. Sometimes your pupils dilate for other reasons. When this happens, it’s referred to as mydriasis. There are many possible causes of mydriasis, including harmless ones like getting your pupils dilated during an eye exam and more serious ones, such as a traumatic brain injury. Anytime you notice that your pupils are dilated without cause, you should contact a healthcare provider. They can figure out what is causing the change and whether you need treatment.

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