The pupil is the black “hole” in the middle of the iris (the colored part of the eye). The pupil looks black because the inside of the eye is dark. Eye doctors dilate the pupil with eye drops to help them view the retina. There are many causes for a dilated pupil outside of the doctors office, however. Some people have a natural anisocoria where one pupil is naturally larger than the other. Plant irritants, pesticides, and antihistamine medications can also make the pupil dilate. Blockage of sympathetic or parasympathetic nerves to the eye can cause a pupil to dilate – there are some serious medical conditions that can cause this blockage such as Adie’s pupil, Horner’s Syndrome, or third nerve palsy. A new onset pupil abnormality needs to be evaluated by an eye doctor.