What causes pupil dilation?

Pupil dilation can be a normal reaction to low light or emotion. When not part of a normal pupil response, pupil dilation can be caused by drugs, medications, poisons, brain injury, or disease.

Medication and drug causes of pupil dilation

A number of medications and drugs can cause pupil dilation including:

  • Amphetamines
  • Antihistamines, such as diphenhydramine (Benadryl, Allermax, Allergia-C) or chlorpheniramine (Chlor-trimeton, Aller-Chlor, Allerlief)
  • Atropine
  • Cocaine
  • Decongestants such as pseudoephedrine (Sudafed)
  • Eye drops such as tetrahydrozoline (Visine)
  • Lysergic acid diethylamide (LSD)
  • Marijuana
  • Methamphetamines
  • Withdrawal from heroin or other narcotics

Poisonous causes of pupil dilation

Pupil dilation can also be caused by exposure to toxins and poisons including:

  • Benzene poisoning
  • Chloroform poisoning
  • Jimsonweed poisoning
  • Toxic mushroom poisoning

Other causes of pupil dilation

Diseases or other conditions involving the nervous system or the eye itself can sometimes result in pupil dilation. Examples include:

  • Adie’s tonic pupil (shrinks to accommodation but poorly to light stimuli)
  • Disorder of the third cranial nerve (also known as the oculomotor nerve, which controls the size of the pupils and most eye movements)
  • Elevated intraocular pressure (excessive pressure inside the eye)
  • Migraine

Serious or life-threatening causes of pupil dilation

In some cases, pupil dilation may be a symptom of a serious or life-threatening condition that should be immediately evaluated in an emergency setting. These include:

  • Brain aneurysm (life-threatening bulging and weakening of the wall of an artery that can burst and cause severe hemorrhage)
  • Brain tumor
  • Cerebral edema (brain swelling)
  • Coma
  • Head trauma
  • Increased intracranial pressure (high pressure inside the skull that is often due to brain swelling or hemorrhage)
  • Intracranial hemorrhage (bleeding inside the skull)
  • Skull fracture
  • Stroke

Questions for diagnosing the cause of pupil dilation

To diagnose your condition, your doctor or licensed health care practitioner will ask you several questions related to your pupil dilation including:

  • When did you (or your friend or family member) first notice your pupil dilation?
  • Do both of your pupils respond the same way to changes in light?
  • What medications are you currently taking?
  • Do you have any other eye disorders?

What are the potential complications of pupil dilation?

Because pupil dilation can be due to serious diseases, failure to seek treatment can result in serious complications and permanent damage. Once the underlying cause is diagnosed, it is important for you to follow the treatment plan that you and your health care professional design specifically for you to reduce the risk of potential complications including:

  • Coma
  • Confusion and mental status changes
  • Permanent disability
  • Seizures
  • Shock
  • Speech problems
  • Vision deficits
  • Weakness or paralysis


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  2. Drug abuse. Medline Plus, a service of the National Library of Medicine National Institutes of Health. http://www.nlm.nih.gov/medlineplus/ency/article/001945.htm.
  3. Ferri FF. Ferri’s Differential Diagnosis, 2d ed. Philadelphia: Mosby Elsevier, 2011.
  4. Kahan S, Miller R, Smith EG (Eds.). In A Page Signs & Symptoms, 2d ed. Philadelphia: Lippincott, Williams & Williams, 2009.
  5. Biousse, V, Newman, NJ. Neuro-Ophthalmology Illustrated. Verlag, Germany: Thieme, 2009.

What is pupil dilation?

The pupil is the circular hole at the very center of your eye, surrounded by the colored area known as the iris. The iris is made up of small muscle fibers that control the size of the pupil, thereby controlling the amount of light allowed to reach the retina, the sensory area at the back of the eye. Normal pupils get smaller, or constrict, with bright light levels, and enlarge, or dilate,... Read more about pupil dilationintroduction


What other symptoms might occur with pupil dilation?

Pupil dilation may accompany other symptoms, which vary depending on the underlying disease, disorder or condition. Pupil dilation with fixed or unequal pupils, as well as unequally sized pupils, are often the most serious symptoms.

Symptoms of drug use or poisoning that may occur along with pupil dilation

Pupil dilation may accompany symptoms of drug use, drug overdose... Read more about pupil dilationsymptoms

Medical Reviewer: William C. Lloyd III, MD, FACS Last Annual Review Date: Sep 20, 2013 Copyright: © Copyright 2014 Health Grades, Inc. All rights reserved. May not be reproduced or reprinted without permission from Health Grades, Inc. Use of this information is governed by the HealthGrades User Agreement.

This Article is Filed Under: Eyes and Vision

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