Symptom
Unequal pupils (anisocoria)
New anisocoria needs prompt assessment
Clinically reviewed · Last reviewed 2026-06-13
When to get urgent help
- A new difference in pupil size that you have not noticed before
- Unequal pupils with a droopy eyelid on the same side
- Double vision, severe headache, or weakness on one side of the body
- A painful red eye with unequal pupils
- Unequal pupils after head or eye injury
Common causes
Some causes are benign; others are urgent. The key question is whether the anisocoria is new or long-standing.
Optic neuritis Sudden vision change in one eye, sometimes with pain on eye movement. Uveitis Inflammation inside the eye can change pupil size and shape. Glaucoma Acute glaucoma can cause a mid-dilated, fixed pupil with pain and halos. Droopy eyelid (ptosis) A smaller pupil with mild droopy lid — can signal nerve or vascular problems. Droopy eyelid (ptosis) A droopy lid with a large pupil and double vision — an emergency. Ocular migraine (visual aura) Temporary visual disturbance that resolves; new persistent anisocoria still needs checking.
Related symptoms
What you can do
- Check old photos if you are unsure how long the difference has been present
- Note any linked symptoms such as droopy lid, double vision or headache
- Seek same-day assessment for new anisocoria rather than waiting
Frequently asked questions
Is it normal to have slightly different pupil sizes?
Yes. A small, stable difference present for years and causing no symptoms is often normal physiological anisocoria. New or changing differences need assessment.
Can eye drops cause unequal pupils?
Yes. Drops for glaucoma, dilation at an eye test, or accidentally getting drop in one eye can temporarily change pupil size. Tell your clinician about any recent drops.
What tests check unequal pupils?
A clinician examines the pupils in light and dark, checks eyelid position and eye movements, and may arrange imaging if a nerve or vascular cause is suspected.