What Causes Small Pupils?

When the pupil becomes small it is said to be constricted. The process of pupil constriction is called miosis. As the body ages, the pupil becomes naturally smaller and does not open as wide in dim light. The average pupil diameter for a 30-year-old is 4.3 millimeters in the daylight and 7.0 millimeters at night. Additionally, there are a variety of disorders that are known to constrict the pupils.


Iritis is the inflammation of the iris and anterior chamber of the eye. Symptoms of iritis include red eye, pain, blurred vision, and photophobia (light sensitivity). Acute iritis, if treated immediately, may heal quickly; however, more chronic cases may not respond well to medication and take months or even years to heal.


Long- or farsightedness is also referred to as hyperopia. This condition occurs when the eyeball is too short or the cornea is too flat. One-fourth of the population is said to suffer from hyperopia. Physicians that specialize in optometry usually offer treatments such as eyeglasses, contact lenses, or laser eye surgery to correct this condition.

Horner Syndrome

Also called Bernard-Horner Syndrome or oculosympathetic palsy, this clinical disorder is caused by damage to the sympathetic nervous system. Aside from constricted pupils, this condition's symptoms include decreased sweating on the affected side of the face, drooping eyelid (ptosis) and sinking of the eyeball into the face. Treatment varies depending on the cause of the nerve damage, but there is no cure for Horner Syndrome itself. Tumors, migraines, injury to major arteries in the brain and stroke can all cause Horner Syndrome.

Drug Use

The use of certain drugs can cause pupil constriction. These drugs include:

  • alcohol
  • opioids
  • atropine
  • LSD
  • mescaline
  • some hallucinogens
  • cocaine
  • amphetamines

Ceasing the use of these drugs will normally allow the pupil to return to normal size 1.