---
title: What causes one pupil larger than the other?
description: "One pupil larger than the other anisocoria can be normal or a sign of disease. Context and associated symptoms matter. ## Benign causes A constant, small difference present for years is often harmless. Some medications also dilate a single pupil…"
url: "https://bluefinvision.com/knowledge-base/topics/symptoms-signs/what-causes-one-pupil-larger-than-the-other/"
updated: "2025-12-31"
category: "Symptoms & Signs"
---

# What causes one pupil larger than the other?

One pupil larger than the other anisocoria can be normal or a sign of disease. Context and associated symptoms matter.

## Benign causes

A constant, small difference present for years is often harmless. Some medications also dilate a single pupil inadvertently.

- Physiological anisocoria with stable appearance in old photos
- Accidental contact with dilating drops or patches
- Past eye trauma affecting the iris muscles
- Long-standing nerve palsy already investigated

## Serious causes of a large pupil

Sudden onset of one enlarged pupil can indicate nerve compression or eye injury. This always needs urgent assessment.

- Third nerve palsy often with double vision and drooping eyelid
- Acute glaucoma causing a mid-dilated fixed pupil and pain
- Head injury with raised intracranial pressure
- Immediate hospital evaluation is essential in new cases