It’s come to the public’s attention that the new A1 Halloween Burger at Burger King will turn your poo Grinch-colored green.

The burger—which channels Japan’s All-Black-Everything Burger—features two black buns stained with food coloring. As the hype for the patty reaches its peak, customers are discovering that the scariest part of the meal isn’t the burger itself. Instead, it’s the side-effect that the food has on the burger-eater’s stool that is truly gruesome.

But can a fast-food burger, even if it has black buns, really turn your poo that strange green color? We reached out to a registered dietician with our question, and asked her to explain the concept of discolored stool for us and our readers.

RD Sarah Romotsky, the Director of Health & Wellness at the International Food Information Council in Washington DC, explains,

“Whenever you see a different color in the toilet it can be a little alarming, but this is actually quite common with certain foods and ingredients. For example, beets can often turn your stool a slight tinge of red or pink because their bright pigment stays largely intact through digestion.”

Okay, that makes sense, but what about green poop? Romotsky explains,

Foods containing high amounts of iron or iron supplements can cause stool to appear very dark, black, or even green. In addition to foods that leave a ‘mark’ on your bowel movements, some food dyes (natural or artificial) can have the same discoloration effect. In all of these instances, it’s important to remember that the foods and ingredients are safe. However, if there is blood in the toilet or color changes that can’t be linked to certain foods or ingredients, I encourage people to consult with a doctor.”

All of you who’ve experienced green poop after eating BK’s newest Whopper can rest assured: you are safe unless you start bleeding from your anus.

[via Twitter, Registered Dietician Sarah Romotsky]