Jump to topic
The iris (the colorful portion of the eye) controls the size of the pupil, which is always changing. Pupils dilate (expand in size) to take in more light in low-light situations and they constrict (get smaller) in bright conditions to minimize the amount of light that enters the eye.
On average, a normal pupil in adults is about two to four millimeters in diameter in bright lights and four to eight millimeters in diameter in the dark. Generally, both pupils are about the same size.
When pupils constrict in direct light, it’s called a direct response. They also constrict when the eye is focused on a near object, which is called an accommodative response. If a pupil does not constrict in light or accommodation or dilate in the dark, it’s considered abnormal.
Mydriasis refers to excessive or prolonged pupil dilation that can happen without any change in light. Note that mydriasis is different from anisocoria, a common condition that affects about 20 percent of the population. Anisocoria happens when the pupils both normally react to light but differ in size by amillimeter or less, on average.
Jump to topic
When pupils dilate, they expand in size. Therefore, pupils look extra big when they’re dilated. The black pupil takes up more of the iris, so the eye may appear darker with less color.
Pupils dilate for many reasons. Here are a few reasons as to why pupils dilate:
Dilated pupils can cause some unpleasant symptoms:
Fortunately, there are some treatment options for larger pupils, including:
Here are answers to some of the most frequently asked questions about dilated pupils.
Pupils are supposed to dilate; it’s a natural pupillary response to low-light situations and to help the eye focus on close-up objects. Pupils may also dilate in response to a variety of stimuli under completely normal circumstances.
However, if the pupils remain dilated or they are unequal in size, seek care from your ophthalmologist. You could have a medical emergency on your hands. Mydriasis and unequal pupil sizes can be signs of serious conditions that affect the brain, such as strokes, tumors, internal bleeding, or head trauma.
Because dilated pupils let light in, if your pupils dilate, you may notice an increased light sensitivity. Dilated pupils also make it harder to read up close.
The pupils shrink in size in brightly lit conditions. They do this to restrict the amount of light that meets the eye. This is a natural protection mechanism.
Many drugs can cause dilated pupils. Here are just a few medications and illegal drugs that can cause dilated pupils:
Academy, Lakeside. “What Kind of Drugs Cause Dilated Pupils?” What Kind of Drugs Cause Dilated Pupils? | Lakeside Academy, boysteenchallenge.mntc.org/what-kind-of-drugs-cause-dilated-pupils/.
Adie Tonic Pupil, www.aao.org/bcscsnippetdetail.aspx?id=1af235eb-71a5-497f-8fac-308c9ea3a0eb.
“Anisocoria and Horner's Syndrome.” Anisocoria and Horner's Syndrome - American Association for Pediatric Ophthalmology and Strabismus, aapos.org/glossary/anisocoria-and-horners-syndrome.
“Dilated Pupil.” Eye Care, www.uclahealth.org/eye/dilated-pupil.
Lick, David J, et al. “The Pupils Are the Windows to Sexuality: Pupil Dilation as a Visual Cue to Others’ Sexual Interest.” Evolution and Human Behavior, www.sscnet.ucla.edu/comm/kjohnson/Lab/Publications_files/Lick,%20Cortland,%20%26%20Johnson%20%282016%29.pdf.
Ou, Yvonne, and University of California. “The Dilated Eye Exam: Why It's So Important.” BrightFocus Foundation, 3 Sept. 2020, www.brightfocus.org/glaucoma/article/dilated-eye-exam-why-its-so-important.
“Polarized Lenses and How They Work.” Dr. Floyd Smith | Optometrist, Westwood, NJ 07675, 31 July 2013, drfloydsmith.com/polarized-lenses-and-how-they-work-3/.
Samant, Monica, et al. “Congenital Aniridia: Etiology, Manifestations and Management.” Expert Review of Ophthalmology, U.S. National Library of Medicine, 2016, www.ncbi.nlm.nih.gov/pmc/articles/PMC6086384/.
Spector, Robert H. “The Pupils.” Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd Edition., U.S. National Library of Medicine, 1 Jan. 1990, www.ncbi.nlm.nih.gov/books/NBK381/.
“Transition Sunglasses Lenses.” American Academy of Ophthalmology, 29 June 2020, www.aao.org/eye-health/glasses-contacts/pros-cons-of-transitions-lenses.
“Why Are My Pupils Dilated? 5 Causes of Dilated Pupils (Mydriasis).” WebMD, WebMD, 6 Aug. 2019, www.webmd.com/eye-health/why-are-pupils-dilated-mydriasis.