A few drops of Visine in someone's drink causes diarrhea.
Some people have heard that slipping someone a "Visine mickey" will give them a bout of sudden, but harmless, diarrhea.
Visine does NOT cause diarrhea. But people have been severely harmed after ingesting Visine as part of a practical joke.
The active ingredient tetrahydrozoline is a vasoconstrictor found in several OTC eye drops (Visine, Murine Tears Plus, etc). Swallowing even small amounts can lead to seizures, cardiovascular collapse, and coma.