The World Anti-Doping Agency (WADA) says allegations that NFL legend Peyton Manning took human growth hormone (HGH) should be investigated.
According to a report by Al Jazeera, the Denver Broncos quarterback was treated with HGH in 2011 while recovering from neck surgery.
However, the news agency's source – Texas-based pharmacist Charlie Sly – recanted the claim since.
Whilst Manning dismissed the report as "complete garbage", WADA says it remains "very concerning".
"Al Jazeera's allegations are very concerning, particularly as it relates to the NFL's and MLB's testing programmes," said David Howman WADA director-general 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 programme.
"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."
No NFL player has tested positive for HGH after a collective bargaining agreement was ratified in 2011 that banned players from using HGH. No players were tested for HGH prior to 2014.
The MLB said that it would investigate Al Jazeera's allegations after it was claimed that several of the league's baseballers took banned hormone supplement Delta-2.