Dilating drops work by blocking parasympathetic receptors in the iris sphincter, allowing the iris dilator to act unopposed and enlarge the pupil. (The pupil dilates mostly because it can't constrict.) This parasympathetic input also controls accommodation, or changing focus of the eye for near objects.

How many drops does it take to dilate your eyes?

Common Duration of Pupil Dilation

Dilating the eyes is a central part of an eye exam. It involves the administration of two or three drops directly into the eye.

What is mydriasis caused by?

When someone's pupils dilate in an unusual way, it is called mydriasis. This may be caused by an injury, psychological factors, or when someone takes certain drugs or medications.