What Causes Dilated Pupils?

Dilated pupils (mydriasis) are when the dark center of your eye is widened. This happens naturally in dim settings. It also happens after taking certain medications or drugs. In both of these situations, dilation is temporary.

Dilated pupils that don't return back to their regular size, however, are not normal. Having one or two pupils that stay dilated could be a sign of a serious medical concern, such as glaucoma or a brain condition.

This article explains why pupils naturally dilate, other reasons they may change size, and when to see a healthcare provider.

Natural Response to Light

Pupils are the round, black circles located in the center of the iris (the colored part of the eye) that allow light to enter the eye.

Pupils constantly change in size, especially in response to light:

• Pupils dilate (become enlarged) typically in low-light conditions to let more light enter the eye.
• Pupils constrict (get smaller) in conditions with higher levels of light to minimize the amount of light that enters the eye.
  

Medications or Drug Use

Some medications cause dilated pupils by interfering with the function of a muscle in the iris (the colored part of the eye) that controls the pupil's size. Some examples include:

• Antihistamines, such as Benadryl (diphenhydramine)
• Muscle relaxants, such as Lioresal (baclofen)
• Decongestants, such as Sudafed (pseudoephedrine)
• Parkinson's medications, such as Symmetrel (amantadine)
• Antidepressants, such as Norpramin (desipramine)

Illicit drugs such as cocaine and LSD can also cause dilated pupils.

Healthcare providers can determine when certain medications or use of other drugs cause dilated pupils. The pupils won't respond normally to light-related stimuli. Most notably, they won't constrict when the eye is exposed to a large amount of light.

Dilated pupils caused by medications or drug use may come with these symptoms:

• Altered mental status
• Dry skin
• Fever
• Flushing
• Myoclonus (sudden, involuntary muscle jerks, shakes, or spasms)
• Seizures
• Urinary retention

In overdosing situations, symptoms might also include:

• Blurred vision
• Heart palpitations
• Shortness of breath
• Constipation
• Urinary retention
• Tremulousness
• Memory difficulty
• Visual hallucinations

Eye Exams

During a comprehensive eye exam, the optometrist or ophthalmologist will likely put drops in a patient's eyes that cause their pupils to dilate.

It takes about 20 minutes to 30 minutes for the drops to take effect. When they do, the eye doctor can test for certain pupillary reactions, which can reveal neurological problems. It can also provide insight into the health of the eye's internal structures, including the retina, vitreous, optic nerve, blood vessels (choroid), and macula.

It typically takes a few hours for the dilating drops to wear off, so it may be a good idea to have someone drive you to your appointment, if possible.

Injuries

Trauma to the eye or the brain can also cause dilated pupils.

Typically, blunt closed trauma damages the iris sphincter muscle (which constricts the pupil) or one of the pathways in the brain that controls it. It can also cause bleeding inside the skull, which may result in dilated pupils.

Other than trauma, eye injuries can also result from:

• Intraocular surgery (cataract removal or corneal transplant)
• Retinal procedures

Medical Conditions

Dilated pupils (in one or both eyes) may also be caused by various medical conditions, including:

• Aneurysm in the brain
• Brain tumor or abscess (like pontine lesions) 
• Excess pressure in one eye caused by glaucoma
• Brain swelling
• Intracranial hemorrhage
• Acute stroke
• Intracranial tumor
• Increased intracranial pressure
• Infection of membranes around the brain (meningitis or encephalitis)
• Migraine headache
• Seizure
• Tumor, mass, or lymph node in the upper chest or lymph node
• Horner syndrome
• Diabetic oculomotor nerve palsy
  

Emotions and Focus

Increased levels of oxytocin (the "love" or "bonding" hormone) may result in dilated pupils in situations involving attraction, mood, or an emotional response to someone or something.

A person's pupils can also dilate in situations where they are concentrating very hard on something, including making a decision.

When to Call a Healthcare Provider

Contact your healthcare provider if you have persistent or unexplained changes in pupil size, especially if the changes are sudden or recent or happen after an injury to the eye or the head.

Get medical treatment immediately if dilated pupils (in one or both eyes) are accompanied by certain symptoms, including:

• Blurred vision
• Double vision
• Eye sensitivity to light
• Fever
• Headache
• Loss of vision
• Nausea or vomiting
• Eye pain
• Stiff neck

A Word From Verywell

The next time you take a look in the mirror, take note of the size of your pupils. You may also want to dim and raise the lights in the room to see how your pupils react.

This will give you a basic idea of what your pupils usually look like and how they typically react to light. This could help you notice if your pupils aren't their usual size or are reacting abnormally.