Tetrahydrozoline is widely believed to cause violent diarrhea if administered orally, as in a few drops of Visine in an unsuspecting person's beverage. However, this is an urban legend, and the actual results may vary from severe nausea and vomiting to seizures or a coma. See Visine Prank. Diarrhea is not a side effect.
Tetrahydrozoline is an alpha agonist and its main mechanism of action is the constriction of conjunctival blood vessels. This serves to relieve the redness of the eye caused by minor ocular irritants.