Uneven Pupils

The causes of uneven pupil size.

I picked up my fifteen-year-old son from a friend’s house last week and to my alarm saw that his left pupil was much larger than the right. The pupil is the dark circle in the center of the eye and it allows light to enter the eye so you can see. Slightly uneven pupils can be normal but my son’s left eye was almost fully dilated meaning almost no blue iris was visible. I quizzed him with several questions as my heart rate increased and my palms became sweaty. Had he hit his head, rubbed anything into his eye, been hit in the eye? No, no, and no were the responses. On our drive to the emergency room he did tell me he had used some contact lens solutions earlier at his friend’s house. Later, after calling around, we finally discovered the name of the drops he had put into his eye -CVS Redness and Allergy Relief drops-not just ordinary contact lens solution.

At the ER the doctor felt comfortable that he had no other signs of head injury so the dilated pupil was likely the result of the drops, especially since he had only put them in that one eye. We were both relieved. Later, Googling, I found many panicked messages from folks who did not know about the dilating effects of anti-allergy drops and who were in various chat threads trying to determine whether their eye problem was serious or not.

In some people one pupil is bigger than the other and there is no obvious reason. This is known as physiological anisocoria and is nothing to worry about. Small differences may occur from day to day, but the doctor should check a large difference in pupil size that persists over time. It is most important that you know your eyes and know when something is out of the ordinary for you.

Here are some of the reasons for one eye having a larger pupil than the other:

Eye Drops

Several eye drops can cause the pupil to become larger. Eye doctors use certain drops during an eye exam so they can get a better look inside your eye. Other common medicines that can cause pupil dilation are anti-allergy medicine and redness relievers. Dilation is only temporary but may last a few hours. In my son’s case it lasted until he woke up the next morning. Most eye drops that have this potential effect say so somewhere on the label on the box.


Certain plants cause dilation of the pupils including Deadly Nightshade and Angels Trumpet, both found in South America. Closer to home, the Yellow Jessamine flower grows in August, Georgia can be a culprit. Rubbing them into an eye could cause one or both eyes to dilate.

Head Injury

Some head injuries can damage the nerves that lead from the brain to the eyes and result in abnormal pupil size. Usually when a head injury is the cause of uneven pupils, there are other symptoms such as a droopy eyelid, loss of vision, fuzzy thinking and other symptoms in the rest of the body. The presence of uneven pupils after a head injury is always a medical emergency.

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