What causes dilated pupils (dilated eyes)?

Dilated pupils are pupils that are larger than normal. They are sometimes called dilated eyes.

The size of your pupil is controlled by tiny muscles in the colored part of your eye (iris) and the amount of light reaching your eyes.

In bright light, your pupils should constrict (get smaller) to prevent too much light from entering your eyes. In dim lighting, your pupils dilate (get larger) to allow more light in.

Normal pupil size generally ranges from 2 to 4 millimeters (mm) in bright light and 4 to 8 mm in the dark or low light. Pupil size tends to get smaller with age.

In one study of people with normal vision, the average pupil sizes under different lighting conditions were:

• 3.04 mm in direct light
• 5.82 mm in near-total darkness

If your pupils are significantly larger, you may have dilated pupils.

Typically, dilated eyes don't respond normally to light. A large pupil that is completely unresponsive to light is called fixed mydriasis, or a "fixed pupil."

Dilated pupils aren't the same as anisocoria, a common condition where each pupil is a different size. Anisocoria affects about 20% of the population and is often harmless (benign), but it can sometimes be a sign of a serious eye or health problem.

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What causes dilated pupils?

Dilated pupil causes include:

Medications

The following prescription and non-prescription medicines can cause your pupils to dilate and affect their ability to react to light:

• Antihistamines
• Decongestants
• Antidepressants
• Motion sickness medicines
• Anti-nausea medicines
• Anti-seizure drugs
• Medications for Parkinson's disease
• Botox and other medications containing botulinum toxin
• Eye drops used to dilate your eyes during an eye exam (tropicamide, cyclopentolate and others), which can also be used to treat uveitis
• Atropine eye drops (used for myopia control and eye conditions like amblyopia)

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Eye injury

An eye injury can damage your iris and cause the pupil to become dilated or abnormal in shape. This can happen if something hits your eye, a chemical splashes onto it or from other kinds of injuries. It can also happen after certain eye surgeries.

Brain injury or condition

A head injury, stroke or brain tumor can affect how your eyes react to light and cause dilated pupils. One or both eyes may be affected.

This is why you see physicians checking an athlete's pupils with a penlight following head trauma sustained during sporting events, or when a patient arrives at a hospital emergency department with other possible stroke symptoms.

Recreational drug use

A number of drugs people use recreationally can cause dilated pupils. These drugs include:

• Amphetamines
• Marijuana
• Cocaine
• LSD
• MDMA (ecstasy)
• Methamphetamine

Research has shown that alcohol can reduce your eyes' ability to recover from exposure to a bright light source (such as oncoming headlights at night) and adapt to changing light conditions. However, alcohol does not cause your pupils to dilate.

Benign episodic mydriasis

This is an unusual but harmless condition where one pupil (unilateral) or both pupils (bilateral) become dilated for a short time. This can be accompanied by blurry vision, light sensitivity or eye pain, but many people don't have other symptoms.

When both pupils are affected, they can be dilated at the same time or alternate between the eyes.

Women and people who get migraines are more likely to get benign episodic mydriasis. On average, each episode lasts around 12 hours, but they can be as short as 10 minutes.

The pupil returns to its normal size and function without treatment.

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Adie's pupil

Also called Adie's tonic pupil or tonic pupil, this is a rare neurological disorder where one pupil or both pupils (less common) are larger than normal and react slowly to light. Sometimes, the pupils do not constrict at all, or react better when you're looking at something up close.

This pupil abnormality may be accompanied by poor or absent tendon reflexes. When this occurs, the condition is called Adie syndrome or Holmes-Adie syndrome.

Generally, the cause of Adie's tonic pupil is unknown. The condition is usually benign and doesn't require treatment, although it can sometimes be associated with trauma, eye surgery, infection or other factors.

Congenital aniridia

This is a rare condition where a person is born with a mostly or completely absent iris, which makes the pupil look very large. It usually affects both eyes and is often accompanied by other eye problems.

Aniridia can make the pupils look dilated, but it isn't true dilation. They only appear that way because the iris is mostly or completely missing.

READ NEXT: What is a blown pupil?

Treatment for dilated pupils

Dilated pupils don't always need to be treated, but when they do, the treatment depends on what's causing them.

If your pupils are dilated or react more slowly to changing light conditions, you may be more sensitive to bright lights. Your eye doctor might recommend eyeglasses that darken automatically in daylight or sunglasses with polarized lenses to make your eyes feel more comfortable outdoors.

Custom prosthetic contact lenses can also help reduce the light sensitivity caused by dilated eyes. Prosthetic lenses give the appearance of having normal pupils of equal size and are especially beneficial for large or irregular pupils caused by trauma.

When to get help

Talk to an eye doctor if your pupils seem dilated or are different sizes. If you recently started taking a new medication, call your prescribing doctor for advice.

In some cases, dilated eyes are a sign of an urgent health problem. Get medical care right away if you recently had an eye or head injury and one or both eyes are dilated.

A stroke can also make one or both pupils dilated. Get immediate medical help if you have sudden weakness or numbness (especially on one side), trouble speaking or balancing, vision changes, or other symptoms of a possible stroke.

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