---
title: "Unequal pupils (anisocoria) | Eye Health Guide"
description: "What causes pupils of different sizes, when longstanding anisocoria is usually harmless, and warning signs that need urgent assessment."
url: https://eyehealthguide.org/symptoms/unequal-pupils/
source: Eye Health Guide
licence: "General information only. Cite the URL and the named medical reviewer."
---

Symptom

# Unequal pupils (anisocoria)

New anisocoria needs prompt assessment

Medical reviewer [Mr Mohamed Mohyudin](https://eyehealthguide.org/reviewers/mohamed-mohyudin/)· Last recorded site review July 6, 2026

### About this review

This page inherits the site’s previous review record. A separate, current article sign-off has not yet been recorded.

[How review works](https://eyehealthguide.org/reviewers/) · [Official biography & credentials](https://mohamedmohyudin.co.uk/author/mohamed-mohyudin/)

Quick answer

Anisocoria means one pupil is larger or smaller than the other. Many people have a small, lifelong difference that is harmless. However, new anisocoria — especially with a droopy eyelid, double vision, severe headache or painful eye — needs prompt medical assessment to rule out serious nerve or blood vessel problems.

## 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

### On this page ⌄

1. When to get urgent help
2. Common causes
3. Related symptoms
4. What you can do
5. Frequently asked questions

## Common causes

Some causes are benign; others are urgent. The key question is whether the anisocoria is new or long-standing.

## 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.

Your next step

## Take this to your appointment

Write down what has changed, when it started, your current medicines and the questions you want answered. Take these to your eye-care appointment.

Sudden loss of sight, severe eye pain or a dark curtain across vision needs immediate assessment. Do not wait to complete a worksheet.

## Sources for this information

These references support the information on this page. Follow the linked source for its full context.

- [Abnormalities of the pupil - College of Optometrists](https://www.college-optometrists.org/clinical-guidance/clinical-management-guidelines/abnormalitiesofthepupil)