The pupils are the black circles, or openings, in the middle of the eyes that allow light to penetrate the eyes. This light is then sensed by light-sensitive cells in the back of the eyes. Pupil dilation is when the size of the pupils increase; the ring of color around the pupils, called the iris, is actually a muscle tissue that expands and contracts to change the size of the pupils to allow more or less light into the eyes.

Is This an Emergency?

If you are experiencing serious medical symptoms, seek emergency treatment immediately.

Light: Natural Cause of Pupil Dilation

The main reason pupils dilate is to assist with vision in dark areas 1. Typically, when you are in a well-lit area, the pupils are very small because there is adequate light reaching the eye. When you step into a dark room, the eyes dilate to allow more light into the eye to provide as clear an image as possible to the brain. You have probably experienced the feeling of the indoors seeming very dark after coming inside during a sunny day. This is because it takes the eyes a little while to dilate and adjust to the reduced lighting.

Mydriatics: Testing the Pupils for Dilation

Mydriatics are medicinal eye drops that cause the eye to dilate. When you get an eye exam, a doctor or technician may use mydriatics to dilate the eyes to better observe them and their response to light. Because the medicine forces eyes to dilate, you may have to wear sunglasses for an hour or so after your exam before your eyes recover; looking around in bright conditions with fully dilated eyes can overwhelm the eyes and cause eye damage.

Drug Use As Causes of Pupil Dilation

Many types of drugs, both legal and illegal, can cause the eyes to dilate. Hallucinogenic drugs such as LSD and mushrooms are known to cause pupil dilation, as are many other substances such as cocaine, marijuana, ecstasy and crystal meth. Prescription antidepressants are also known to cause dilation in the eyes.

Poison: Cause of Pupil Dilation

Pupil dilation is a common response to being poisoned by different kinds of plants and chemical poisons. The black locust tree has bark that can cause poisoning that leads to pupil dilation. Many toxic chemicals such as chloroform and jet fuel will cause pupil dilation if accidentally ingested.

Arousal & Pupil Dilation

Because the nervous system controls the muscles of the irises, the response of the nervous system to different stimuli results in involuntary pupil dilation. Another commonly cited reason the pupils dilate is in response to excitement or sexual arousal. When a person sees something or someone he finds very attractive, his eyes may dilate.