According to Dr. Ian Lustbader, a gastroenterologist and clinical associate professor of medicine at New York University's Langone Medical Center in New York, the food dye used in the Oreos isn’t broken down by the body during digestion.
He told Live Science that it takes more than just one or two of the cookies to have this effect – and the sugar and fat contents of having “a bunch” of the cookies is probably more unhealthy that the food dye.
An Oreo spokeswoman said:
The Oreo Peep limited-edition sandwich cookies contain a bright pink food coloring, FD&C Red Number 3. We use this ingredient in the product consistent with U.S. Food and Drug Administration (FDA) regulations.
Certain foods can temporarily color the tongue and inside of the mouth. This is common with brightly colored products such as ice pops, gelatins and drink mixes, as well as with certain fruits and vegetables, including beets and cranberries. Such foods can also temporarily color stool.