Burger King has an ongoing seasonal promotion wherein it offers a Halloween-themed Whopper cheeseburger with a black bun and a special steak sauce. According to a report from Gizmodo, however, the burger has been wreaking havoc on peoples’ digestive systems, and turning their poop green.

There has been much speculation surrounding the strange phenomenon of a cheeseburger going in black and coming out green. Burger King has released a list of the burger’s ingredients, which the company reiterates are all approved by the FDA.

The dye used to turn the burger’s buns black was the primary culprit, as black dyes are often made from green dye. The dye used in the bun is a concoction of Yellow #6, Red #40, caramel color, and Blue #1, which is also referred to as Brilliant Blue FCF.

This color is notorious for not getting along well with the intestines, meaning it gets passed off with the rest of the body’s waste. The catch is, however, that the dye doesn’t always look blue on the other side.

Brilliant Blue FCF is often combined with a yellow-hued dye called tartrazine, which results in a green dye. This can combine with bile in the stomach to make poop appear green in color. Whether it’s Brilliant Blue FCF or another type of dye, the blue in the Halloween Whopper is likely what’s turning so many people’s stool green.