People who tried Burger King’s Halloween Whopper were a bit surprised after discovering their poop has turned green.

Burger King’s black Halloween Whopper has a really unfortunate side effect: http://t.co/C3DOu1MEEt pic.twitter.com/Eb8SNjRKBy

— E! Online (@eonline) October 7, 2015

The hastag #GreenPoop became a trending topic on Twitter as customers of Burger King shared their experience after eating their limited edition burger. Others posted pictures of their stool while one Twitter user described the color as “comical green.”

I’ve never had the urge to eat @BurgerKing, until I found out the new whopper turns your poop a shade of comical green. — Taylor Trammell (@taylorttrammell) October 6, 2015

.@BurgerKing I’ve made a new graphic for your Halloween Burger. You’re welcome. #GreenPoop pic.twitter.com/IcIVemOiPX — Giana Mucci (@RatedGiana) October 6, 2015

One tweeter noticed that because of this mystery, people are now posting Instagram pictures of their food “after they ate it.”

Well @BurgerKing what’s up with my #GreenPoop from your #HalloweenWhopper? You put that many chemicals in?#burgerking pic.twitter.com/fHpFgTxQzz — Brandon Polk (@NiceGuyBrandon) October 5, 2015

Other Burger King customers vowed to never eat the black burger again while there are those who wanted to try it just to see how their stools will look like.

Burger King’s sales of Halloween burgers are either going to go down or through the roof with people wanting comical green poop ?

— Laura (@laurajones493) October 6, 2015

According to ABC, a spokesperson for the food chain revealed that the Halloween Whopper buns contain A1 Steak Sauce which is responsible for its black hue. The spokesperson also pointed out that they used less than one percent of food dye for the bun. The limited edition burger has already been out in Burger King chains in Japan, however, unlike its American counterpart, they use bamboo charcoal and squid ink.

It’s a Halloween miracle. A.1. #HalloweenWhopper https://t.co/7dBr4CYqkV

— Burger King (@BurgerKing) October 5, 2015

There is nothing to be worried about as Burger King assured that the food coloring and flavoring for the controversial burger are “common in industry” and passed the Food and Drug Administration safety standards. Moreover, those who have experienced a ghoulish green poo, did not say if the burger made them feel unwell.

A greenish stool is usually results from eating vegetables and change in diet. Mayo Clinic also confirmed that green food coloring is known to give one’s stool a green shade. This could mean that the dye used for the food was not digested properly. One tweeter wrote, “it will haunt your colon” then added the hashtags #ScaryWhopper and #greenpoo.

Try the new Halloween Whopper at Burger King! It will haunt your colon. #ScaryWhopper #greenpoo

— Tom Brandt (@tomsbrandt) October 1, 2015