The World Anti-Doping Agency (WADA) on Monday described allegations made in an Al-Jazeera report that Denver Broncos quarterback Peyton Manning took human growth hormone (HGH) as "very concerning" and has urged "increased collaboration" with sports leagues in the United States.
Though Manning has vehemently denied the allegations and the source of the Al-Jazeera report has since recanted the claims he was shown making in the film, WADA says that it expects a careful investigation by the relevant authorities.
The Dec. 27 investigative documentary The Dark Side: The Secret World of Sports Doping linked well-known players from the National Football League (NFL) and Major League Baseball (MLB) with performance-enhancing drugs.
"Al-Jazeera's allegations are very concerning, particularly as it relates to the NFL's and MLB's testing programs," David Howman, director general of WADA, said in a statement.
"While the NFL and the MLB are not signatories to the World Anti-Doping Code, in recent years WADA has been working with them and other professional leagues in the United States to try to bring them closer to WADA's program.
"In particular with the NFL, we have been offering guidance to enhance, and increase the transparency of, their testing program. We would, of course, welcome increased collaboration with the leagues and their players' associations to discuss appropriate enhancements that could be made in support of clean athletes."
The Al-Jazeera report claimed that Manning took human growth hormone following neck surgery in 2011, and on Sunday the football player acknowledged he visited a clinic that allegedly supplied the banned substance.
However, Manning, who missed the entire 2011 National Football League season with a serious neck injury, strenuously denied the claim made by the news network's report that he had used human growth hormone.
Green Bay Packers linebackers Clay Matthews, Julius Peppers and Mike Neal, who were also linked with human growth hormone use in the report, have strenuously denied any wrongdoing.
"As it relates to the particular allegations by Al-Jazeera, WADA expects that they will be carefully investigated by the relevant authorities and that, if warranted, necessary and appropriate steps would be taken," Howman said.
The NFL collective bargaining agreement, ratified in 2011, banned human growth hormone but players were not tested for the banned substance until 2014. No NFL player has tested positive for HGH.
Major League Baseball said it would investigate allegations made in the documentary that several of its players took banned hormone supplement Delta-2.
Among those baseball players named in the Al-Jazeera report as having received supplies of Delta-2 were Ryan Howard of the Philadelphia Phillies and Ryan Zimmerman of the Washington Nationals.
A lawyer for Howard and Zimmerman denied the claims.
In a follow-up email to Al-Jazeera, Sly said that when he was speaking on camera, he was "in no state of mind to be making any coherent statements as I was grieving the death of my fiancee."