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Tetryzoline (INN; also known as tetrahydrozoline), a derivative of imidazoline, is found in over-the-counter eye drops and nasal sprays. Other derivatives include naphazoline, oxymetazoline, and xylometazoline.
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.
An urban legend suggests that tetrahydrozoline can cause violent diarrhea if administered orally, such as by putting a few drops of Visine in an unsuspecting person's beverage. However, the actual results of the prank may be worse, varying from severe nausea and vomiting to seizures or a coma. Diarrhea is not a side effect.
In popular culture
Tetrahydrozoline was used as a plot device in the 1990 film The Spirit of '76. Time travelers from the year 2176, where tetrahydrozoline is a rare commodity, have traveled back to 1976 and have to use tetrahydrozoline eye drops, a common item in 1976, as part of a battery in a time machine in order to return to the future.