What causes dilated pupils?

Dilated pupils not resulting from a normal response to low light are typically caused by drugs, medications, poisons, or brain injury or disease.

Medication or other drug causes of dilated pupils

A number of therapeutic medications, as well as illicit drugs, may cause dilated pupils including:

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

Poisonous causes of dilated pupils

A number of biological or chemical toxins and poisons can lead to dilated pupils. These include:

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

Other causes of dilated pupils

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

  • 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)

Serious or life-threatening causes of dilated pupils

In some cases, dilated pupils may be a symptom of a serious or life-threatening condition that should be immediately evaluated in an emergency setting. Examples 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)
  • 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 dilated pupils

If your dilated pupils are not associated with a medical emergency, your doctor or licensed health care practitioner will ask you several questions to diagnose your condition including:

  • When did you (or your friend/family member) first notice your dilated pupils?
  • 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 dilated pupils?

Many of the complications of brain injury or damage, which are often accompanied by dilated pupils, can be serious or even life threatening. Some of these serious complications include:

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


  1. Anisocoria. Medline Plus, a service of the National Library of Medicine National Institutes of Health. http://www.nlm.nih.gov/medlineplus/ency/article/003314.htm.
  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.

What are dilated pupils?

The pupils look like round, black dots in the center of the eyes, but they are actually openings that allow light to enter the eyes. The size of the opening is controlled by the iris, the colored portion of the eye surrounding the pupil. The pupils constantly change size in response to light, to the eye’s focusing distance, and to emotions. Normally, the pupils dilate (enlarge) in darker condit... Read more about dilated pupilsintroduction


What other symptoms might occur with dilated pupils?

Dilated pupils may be accompanied by other symptoms, depending on the cause. Serious injuries may produce pupils that are unequal in size. Dilated and fixed pupils, as well as pupils of unequal size, are the most worrisome symptoms.

Symptoms of drug use or poisoning that may occur along with dilated pupils

Dilated pupils may accompany other symptoms of drug use, drug ov... Read more about dilated pupilssymptoms

Medical Reviewer: Cynthia Haines, MD Last Annual Review Date: Aug 1, 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

Popular Eyes and Vision Slide Show