MIAMI, Oct. 6 (UPI) -- Burger King customers sampling the new Halloween Whopper said the burger's black bun carried an unexpected side effect -- it turned their poop green.
The fast food chain unveiled the Halloween Whopper last month, saying the bun's black color comes from A1 steak sauce, but some customers posting on social media said another, unknown ingredient is believed to be the cause of their feces turning a bright green.
"Yesterday I had the Halloween Whopper... Today my poop is green. Yesterday was the last time I'll have the Halloween Whopper," @Danny_Seth tweeted.
Not all customers were dissatisfied with the side effect.
"Took like three days for my poop to stop being green, but was totally worth it, @BurgerKing" @ImHunterStanton wrote on Twitter.
The hashtag "#greenpoop" was trending Tuesday on Twitter.
The company did not specify whether the bun contains Green #3, a food dye known to cause green-colored stool. The color is also known to result from food not being broken down quickly enough by bile.
"There have been reports that the Halloween Whopper turns your poop green. I can confirm those reports," @itsmartinryder tweeted.