The primary reason pupils enlarge is to allow more light into a person's eye when in a dark room or area, according to Healthgrades. When people go in for certain eye exams, the eye doctor uses drops to dilate or enlarge the pupils to examine the eyes better.
Cough and cold and decongestant medicines often cause a person's pupils to enlarge, according to Healthgrades. Cocaine and marijuana also cause pupil enlargement; however, the enlargement caused by drugs typically goes away after the effects dissipate. Health conditions, such as a stroke or tumor, can cause enlargement. Unlike a drug-induced enlargement, these medical triggers may impact only one pupil.