---
title: What causes unequal pupil sizes?
description: "Unequal pupil sizes anisocoria can be harmless, but sometimes indicate serious eye or brain problems. ## Common and less serious causes Many people have a small, stable difference between pupil sizes that they were born with. Certain eye drops or…"
url: "https://bluefinvision.com/knowledge-base/topics/symptoms-signs/what-causes-unequal-pupil-sizes/"
updated: "2025-12-31"
category: "Symptoms & Signs"
---

# What causes unequal pupil sizes?

Unequal pupil sizes anisocoria can be harmless, but sometimes indicate serious eye or brain problems.

## Common and less serious causes

Many people have a small, stable difference between pupil sizes that they were born with. Certain eye drops or past eye surgery can also affect pupil size.

- Physiological anisocoria where the difference is usually under one millimetre
- Old trauma to the eye or iris
- Medications or eye drops that dilate or constrict one eye
- Previous inflammation affecting the iris muscle

## Serious causes needing urgent review

Sudden anisocoria with other symptoms can signal nerve damage, brain problems, or acute glaucoma. Prompt assessment is crucial.

- New unequal pupils with drooping eyelid, double vision, or headache
- Large fixed pupil after head injury or severe eye pain
- Associated weakness, speech difficulty, or confusion
- Any rapid change should be treated as an emergency until checked