Halloween is just around the corner, and to celebrate, Burger King has introduced a black-bunned Whopper with A1 streak sauce in the United States.

But it turns out that this special treat comes with a surprising trick — ghoulishly green poop.

Not long after Burger King dropped the limited-edition Whopper, its ghastly effects began to surface.

People began reporting their green poops on Twitter.  

Soon the spooky stool was trending under hashtag #greenpoo.

So what's behind the emerald excretions? 

Probably just food colouring.

Among the dyes used to make the black bun is Blue #1 — also called Brilliant Blue FCF — which doesn't absorb well in the intestines and is often combined with the yellow coloured tartrazine to make green dye, Gizmodo reports.

That said, Burger King tells ABC that the dark bun's color comes from the "smoky black pepper flavor of A.1. baked into the bun," and that it contains less than one percent food dye.

Other possible causes of green stool include bile, which can appear when the poop moves hastily through the intestinal tract, and sometimes from ingesting iron supplements, the Washington Post reports.

So far Burger King is staying pretty quiet about the fearful feculence.

The company's nutritional guidelines say the dyes used in the black buns are "commonly used in the industry and within the safe and Acceptable Daily Intake approved by the Food and Drug Administration."

So there's little to fear.

Or is there? 

After all, the burger also boasts a whopping 710 calories, 43g of fat and 1530 mg of sodium.

Now that's scary.