Americans who decided to try Burger King's new black Halloween Whopper this week were in for a bit of a surprise when it came out on the other side.

Customers said the Whopper was turning their poop a bright 'comical' green, some even joking that the fast food chain should bring back the special edition burger for St. Patrick's day.

'GreenPoop' became a popular hashtag on Twitter, with some users even sharing pictures of their bathroom surprise.

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Americans who have tried Burger King's new black Halloween Whopper this week said the burger left them with poop was a shocking hue of bright green

One customer described the hue to be 'cartoon' and 'comical green' and many took to Twitter to share that they too had #GreenPoop, even including pictures of the neon-colored surprise

'Thanks to the black Whopper, for the first time, people are Instagramming their food the day AFTER they eat it!', wrote one user.

Some people swore they would never eat the burger again, but many were excited of the prospect of seeing just what the burger would do to their stool.

'I ate a Halloween Whopper just to see if my poop turned green and it DID,' wrote one user.

'Don't rly [sic] care how it tastes, I just want that Halloween burger from Burger King for the rad green poop,' tweeted another.

Some were less than amused with the new hue of their stool, but others defended the limited edition burger, green poop and all

One user said it took three days for his poop to stop turning green but that it was 'totally worth it'.

The Halloween Whopper's black bun has already been seen in the franchise's stores in Japan, which use squid ink and bamboo charcoal to give the burger its black color.

But the American version, which went on sale for $4.99 on September 28, receives its dark hue from the A.1. steak sauce that's baked into the bun, a Burger King spokesperson told ABC.

The spokesperson added that the bun contains less than one percent of food dye.

The news even intrigued others on Twitter to try the burger, just so they could experience it for themselves

Burger King's nutrition guide also states that the flavoring and food coloring for the Halloween Whopper are 'commonly used in the industry' and meet the Food and Drug Administration's safety guidelines.

Green food dye has been known to turn one's poop bright green, according to the Mayo Clinic.

No matter the cause, the green poop phenomenon has clearly peaked people's interest in the limited edition burger.

'I had absolutely no interest in the Halloween Whopper until I learned it could give me green poop,' admitted one tweeter. 'Now I'm intrigued.'