What Does It Mean When One Pupil is Bigger than the Other?
Medically reviewed by Andre Stone | MD, University of Pittsburgh School of Medicine on August 25th, 2026.
When one pupil is bigger than the other, called anisocoria, it can indicate various conditions ranging from harmless to serious. Pupil size varies with lighting, age, and other factors. A small, longstanding difference between the pupils is common and may be a harmless natural variation, although a larger or newly developed difference can have a medical cause. Common causes include eye injuries, certain medications, neurological conditions like Horner's syndrome, or brain issues affecting the third cranial nerve. Sudden onset anisocoria, especially with symptoms like severe headache, vision changes, or drooping eyelid, requires immediate medical evaluation as it may signal stroke, aneurysm, or increased brain pressure.
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