At 6-foot-4, he walked with the swagger of a successful high school athlete — even if his pale, fleshy physique no longer looked much like a figurine atop a trophy. He spoke confidently of the minutiae of sports doping, including NFL-banned hormone Delta-2 (or D-2), HGH, Adderall, Dexedrine, Toradol, Ipamorelin and how long it takes testosterone to leave the human body.

And, unknowingly part of Al Jazeera America’s controversial investigation of performance-enhancing drugs, a man the network identified as “doctor of pharmacy” Charlie Sly wasn’t shy about sharing his knowledge — and pushing his products.

“I can give you something to use right now,” Sly told Liam Collins, reportedly pushing steroids on the British hurdler Al Jazeera had go undercover. “… There’s a bunch of football players who take this. And a bunch of baseball players who take it, too.”

Sly — if his recorded statements accurately convey his views, perhaps the least discreet advocate of performance-enhancing drugs ever — also opened up about his alleged clients.

On Dustin Keller, formerly of the Miami Dolphins: “We just used Delta-2 because it wasn’t detectable.”

On Ryan Howard of the Philadelphia Phillies: “He knows to take stuff twice a day. … I think [he saw] maybe more explosiveness [with D-2].”

On Ryan Zimmerman of the Washington Nationals: “I worked with him in the off-season … [He thinks D-2] does its job.”

On James Harrison of the Pittsburgh Steelers: “He’s a f—ing beast. … Yeah, he takes [D-2].”

On Mike Neal of the Green Bay Packers: “Last year, I went out to Green Bay for like six weeks. I set Mike’s stuff up, but then, like, he started bringing everybody. I’m not even joking. More than half the team started to come by.” Other clients, according to Sly: Green Bay’s Julius Peppers and Clay Matthews.

And, most startlingly, Sly alleged football icon Peyton Manning, currently of the Denver Broncos, took HGH prescribed to his wife: “Another time that I worked with Peyton, him and his wife would come in after hours and get IVs and s—,” Sly said.

In a story as explosive as this, recounting the denials, responses and non-responses takes a while. Manning, Howard, Zimmerman and Harrison denied Sly’s allegations.

An attorney for Howard and Zimmerman denied the claims in a statement to ABC Action News in Philadelphia. “It’s inexcusable and irresponsible that Al Jazeera would provide a platform and broadcast outright lies about Mr. Howard and Mr. Zimmerman,” the attorney, William Burck, said.

In a statement, the Green Bay Packers denied the allegations on behalf of its players. Peppers and Matthews were quoted at PackersNews.com denying any connection to the drugs. “Peppers said he was a ‘little bit shocked’ to learn of the allegations and did not know anybody involved with Sly,” the site reported.

“ ‘It’s not true,’ Peppers said. ‘It’s completely erroneous, and I think it’s irresponsible journalism, in my opinion. I’m subject to the same steroid and drug-testing policy as everybody else. So I don’t understand how I could be linked to something like this,’ ” PackerNews reported.

Neal, who served a four-game suspension before the 2012 season for violating the NFL’s policy on performance-enhancing drugs, “neither confirmed nor denied allegations made against him, electing to not comment on the report directly,” according to PackerNews. “You might as well stop asking me questions,” Neal said.

Keller did not respond to Al Jazeera’s request for comment. The Lafayette Journal and Courier identified Sly as one of Keller’s “best high school friends” in 2008, who was present when Keller was drafted that year, and newspaper reports confirm they played basketball together in high school.

Meanwhile, Sly — a former unpaid intern at the Indianapolis-based Guyer Institute of Molecular Medicine, which treated Manning, as The Washington Post’s Barry Svrluga and Mark Maske reported — declared himself an unreliable narrator.

“It has come to my attention that the broadcaster Al Jazeera has somehow obtained recordings or communications of me making statements concerning a number of athletes, and that Al Jazeera plans to air a program about them,” Sly said in a denial posted to YouTube. “Any recordings of me were made without my knowledge or consent. It is my belief that an individual named Liam Collins secretly made those recordings. Liam is a recruited fraudster who is banned in his native United Kingdom from running any investment businesses. The statements on any recordings or communications that Al Jazeera plans to air are absolutely false and incorrect.”

He added: “To be clear: I am recanting any such statements and there is no truth to any statement of mine that Al Jazeera plans to air. Under no circumstances should any of those statements, recordings or communications be aired.”

No one answered calls from The Post to numbers for Sly in public records; Indiana public records confirmed that he was a licensed pharmacy intern in the state from 2010 to 2013.

In an interview on Al Jazeera, one of the story’s investigative reporters, Deborah Davies, responded: