7 reasons why your pupils may be dilated, from low light to sexual attraction

dilated pupil meaning — Pupils often dilate in response to low light.
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Dilated pupils mean that your pupils appear larger in size.
The most common reason for dilated pupils is low light in a dark room since lower light causes your pupils to grow.
Dilated pupils are also caused by drug use, sexual attraction, brain injury, eye injury, certain medications, or benign episodic unilateral mydriasis (BEUM).
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The size of your pupils can say a lot about your health and your state of mind.

In most cases, pupils change size based on the light and dilate, meaning that appear larger, when you’re in a darker room. But your pupils may also change size based on injuries, drug use, or even sexual arousal.

Here are seven reasons your pupils may be dilated, and when you should see a doctor.

1. Light

Your pupils help you see by controlling how much light enters your eye. To do this, they react to the amount of light around you – brighter light makes your pupils grow smaller, while low light makes your pupils grow larger.

Pupil size is controlled by the brain, which receives signals from each retina and sends a message for your eyes to adjust to the light. Muscles in the colored part of your eye, your iris, then work to constrict or dilate your pupils, letting in more or less light as needed.

2. Medication

"Certain medications can interfere with the messages from the nerves that tell the pupil to dilate or constrict," says Devin D. Mackay, MD, Director of Neuro-Ophthalmology at the Indiana University School of Medicine.

There are many different types of medications that can interfere with the nerves that make your pupils dilate, including:

Antihistamines like diphenhydramine (Benadryl) and loratadine (Claritin)
Motion-sickness patches like scopolamine
Stimulant medications like (amphetamine) Adderall or methylphenidate (Ritalin)
Anticholinergic medications used to treat overactive bladder or Parkinson's disease like Oxybutynin (Ditropan XL, Oxytrol) and benztropine (Cogentin)

3. Recreational drug use

Recreational drugs also affect your nervous system and can cause your pupils to dilate or shrink regardless of the light you're in. Some common drugs that affect your pupils include:

Cocaine and crack cocaine can cause your pupils to dilate because your body releases higher levels of endorphins and adrenaline, which stimulate the nerves that control your pupil size.
MDMA, also called molly or ecstasy, can make your pupils dilate because it increases serotonin, dopamine, and norepinephrine hormones in your body. These chemicals stimulate your nervous system, including the nerves that control your pupil.
Hallucinogens like LSD (lysergic acid diethylamide) and psilocybin mushrooms dilate your pupils by increasing your serotonin levels and activating your nervous system.

For a full list of drugs that can dilate your pupils, see here.

4. Brain injury

The nerve fibers that control the size of your pupil are within a larger nerve that originates in your brainstem called the oculomotor nerve. When this nerve is injured, it can cause many different problems in your eyes.

"Severe head trauma can stretch the nerve and damage its fibers, which can result in a dilated pupil," Mackay says. Damage to the oculomotor nerve can also make your eyelid droop or limit your eye movement on the side of the injury Mackay says.

Brain injury can be caused by something hitting your head, but you can also have an internal injury from a stroke, a brain tumor, or bleeding in your brain. This type of injury can cause your brain to swell and press on the oculomotor nerve, making one or both of your pupils dilate, Mackay says.

5. Sexual attraction

When you feel sexually attracted to someone, your nervous system is stimulated and your body releases several hormones including dopamine, norepinephrine, and serotonin.

When these chemicals are released, your pupils dilate, even if you're just looking at a photo of an attractive person.

6. Eye injury

The colored part of your eye, the iris, has muscles that control the constriction and dilation of your pupils. "Trauma to the eye can cause damage to the muscles that constrict the pupil, which makes the pupil stay dilated," Mackay says.

Injuries that can damage your iris muscles and dilate your pupils include:

A blunt force hitting your eye
An object penetrating your eye
A chemical burn of your eye
An eye burn from a fire

If the injury bruises your iris muscles, your pupils will likely be dilated temporarily, but if your iris muscles are torn or otherwise damaged, the change may be permanent.

7. Benign episodic unilateral mydriasis

Benign episodic unilateral mydriasis (BEUM) is a condition in which the pupil in one of your eyes spontaneously dilates. It can stay dilated for just a few minutes or can go for days before returning to its normal size, Mackay says.

"There is likely not a single underlying cause," Mackay says, "But it can happen from underaction of the constricting muscle or overaction of the dilating muscle of the affected pupil."

Usually, BEUM is nothing to worry about. "The condition is frequently associated with migraine, but as the name implies, it is benign and does not cause any lasting damage or disability," Mackay says.

Insider's takeaway

There are many reasons your pupils may be dilated, and in many cases, there is nothing to worry about.

But because there are some causes that are harmful, "a suddenly dilated pupil should be evaluated urgently, preferably by a qualified eye care provider," Mackay says.

In particular, if one or both of your pupils are dilated and you have other symptoms like a drooping eyelid or double vision, you should get medical attention as soon as possible, Mackay advises.