LAS VEGAS, United States (AFP) — Juiced-up athletes gathered in Las Vegas on Sunday for the first-ever Enhanced Games, where organisers predict world records will be unofficially “beaten” by competitors using performance-enhancing drugs that have sparked health concerns.

Around 40 sprinters, swimmers and weightlifters have spent the past four months in Abu Dhabi taking combinations of testosterone, human growth hormone, peptides and anabolic steroids — all banned by events like the Olympics.

The event, starting at 3:35 pm (2235 GMT) and streaming on YouTube and Roku, has been denounced by athletics governing bodies and anti-doping agencies as dangerous and against the spirit of sport.

But participants, lured by prize money of up to $1 million for beating world records, will include Olympic medallist swimmers James Magnussen, Cody Miller and Ben Proud, who have all taken drugs.

Hafthor “Thor” Bjornsson, best known for playing “The Mountain” in “Game of Thrones,” will try to break his own deadlift record.

Former 100m sprint champion Fred Kerley will be one of the few athletes competing without drugs at the event.

Max Martin, chief executive officer and co-founder, has predicted multiple world records will be “beaten,” though the feats will not be officially recognised.

Weightlifters Beatriz Piron and Arley Mendez surpassed world records in training, he told a press conference on the eve of the event.

“Hopefully they’ll be able to do it tomorrow as well, and then we’ll see a few more,” said Martin.

“My guess is we’ll see quite a few.”

Swimmers will also be allowed to wear the types of “supersuits” that led to many world records falling around the 2008 Beijing Olympics, but were subsequently prohibited.

Chief sporting officer Rick Adams said that he respects that “specific international organisations” will not accept any records set on Sunday, even if they are broken by clean athletes like Kerley.

“But it is uncontroverted that if one of our 50m freestyle… moves through that water in less than 20.88 seconds, they have moved through that water faster than any other human being in history,” he said, referring to Cam McEvoy’s official record.

The Enhanced Games are taking place at a purpose-built $50 million arena in a Las Vegas casino parking lot, a structure to be dismantled hours after the final race.

In a marriage of sports, biohacking, politics and entertainment, investors including billionaire Peter Thiel are rumored to be attending, while rock band The Killers will play a closing set. Donald Trump Jr is among the event’s investors.

Health experts warn that several of the substances could risk “life-shortening and fatal consequences,” including heart, liver and kidney issues, as so little is known about their long-term effects.

But Enhanced Games officials say all medications are approved by the US Food and Drug Administration, and no expense has been spared on hiring top medical experts.

Emergency officials will be standing by in case something goes wrong.

“I’m reasonably confident nothing will happen,” said chief medical officer Guido Pieles.

Pieles described the risk of the medications as “clearly there” but “really manageable.”

His team has published the percentage of athletes taking each substance, with Martin vowing that “transparency is core to our DNA.”

Yet athletes said that they are not able to disclose the specific combinations each has taken.

“The reason for that is that actually it doesn’t matter,” Martin told AFP, noting that combinations are tailored to each athlete.

Also, parent company Enhanced — which sells many of the substances being taken by its athletes to the public — does not want to “advertise for certain protocols to create imitation potential,” said Martin.