These mismatched pupils are from a routine eye exam, not the cocaine eye-drops used to check for Horner syndrome.

Cocaine has been used recreationally for hundreds of years, and medicinally for just as long. It was even a founding ingredient of Coca-Cola. While you can’t get it in soft-drinks anymore, it is still used clinically in a few different ways.

Cocaine is primarily used as a topical anesthetic as well as a vaso-constictor, so it can numb an area before a procedure and decrease bleeding and swelling after. In certain cases, cocaine eye-drops are used to check for a problem with the sympathetic nervous system (i.e. fight-or-flight). Because cocaine directly activates the sympathetic nervous system, if the eye-drops don’t cause pupil dilation, then there might be a problem somewhere in that pathway (called Horner syndrome). WebMD says that common cocaine side-effects include an unusual sense of well-being. Not surprisingly, in its purified form, cocaine is one of the most addictive substances around.

You can see how dilation of the left pupil makes it easier to examine the back of the eyeball. Unfortunately, it also makes it harder to ride your bike home from the clinic!