Which pupil is the problem?

It depends on lighting. A larger difference in bright light often points to the larger pupil. A larger difference in dim light can point to the smaller pupil.

Can eye drops cause anisocoria?

Yes. Dilation drops, motion sickness patches, and some sprays or medicines can affect one pupil.

Why do old photos help?

Old photos can show whether the pupil difference has been there for years. That can prevent unnecessary testing.

Condition

Anisocoria

Q: Should I go to the emergency room?

Go now if unequal pupils are sudden and come with severe headache, neck pain, double vision, droopy eyelid, injury, or neurologic symptoms.

Also known as Unequal Pupils, Different Pupil Sizes, One Large Pupil, One Small Pupil

Updated May 19, 2026·For educational purposes only. Not a substitute for medical advice. See our terms.

Bottom Line

Anisocoria means the pupils are different sizes. It is often harmless, but sudden unequal pupils with headache, droopy eyelid, double vision, or weakness need urgent care.

Anisocoria means one pupil is larger than the other. A small, stable difference can be physiologic, which means normal for that person ¹.

The exam compares pupil size in bright and dim light. It also checks eyelids, eye movement, vision, pain, medicines, eye drops, trauma, and old photos.

Some causes are urgent. Horner syndrome, third nerve palsy, eye trauma, and certain medicine exposures need fast evaluation when symptoms are new or severe ².

Common Causes

Common causes include physiologic anisocoria, eye drops, Adie pupil, eye inflammation, past eye surgery, trauma, Horner syndrome, and third nerve palsy.

Physiologic anisocoria is a small pupil difference that stays stable. Horner syndrome often has a smaller pupil and a mild droopy eyelid ³.

How It Is Diagnosed

The doctor measures both pupils in light and dark rooms. They also test vision, eye pressure, eyelids, and eye movement.

Special drops may help separate Adie pupil, medicine effects, Horner syndrome, and other causes. Imaging may be needed when a nerve problem is suspected ².

When To Seek Care

Get urgent care for sudden unequal pupils with severe headache, neck pain, double vision, droopy eyelid, injury, or vision loss.

If the pupil difference is old, small, and unchanged, it is usually less urgent. Bring old photos to the eye visit.

Common Questions About Anisocoria

Yes. A small, stable difference can be normal for some people ¹.

Next Steps

1Call 911 for unequal pupils with face droop, slurred speech, weakness, confusion, or trouble walking.
2Go to the emergency room for sudden severe headache, neck pain, double vision, droopy eyelid, trauma, or vision loss.
3Book an eye exam for a new pupil difference without emergency symptoms.
4Bring old photos, a full medicine list, and any eye drops or patches you used.
5Do not drive if you have new double vision or sudden vision loss.

Find specialists for Anisocoria

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