According to a report by SB Nation Mayweather allegedly received an intravenous injection of saline and vitamins that was banned under World Anti-Doping Agency guidelines, on the eve of his megabout with Manny Pacquiao in Las Vegas on May 2.
However, Mayweather said in a statement that he "did not commit any violations" and he was fully supported by the United States Anti-Doping Agency (USADA) earlier on Thursday.
"As already confirmed by the USADA statement, I did not commit any violations of the Nevada or USADA drug testing guidelines," said Mayweather, who beat Pacquiao 48-0.
"I follow and have always followed the rules of Nevada and USADA, the gold standard of drug testing.
"Let's not forget that I was the one six years ago who insisted on elevating the level of drug testing for all my fights. As a result, there is more drug testing and awareness of its importance in the sport of boxing today than ever before."
According to the SB Nation report, USADA collection agents visited Mayweather's house in Las Vegas the night before his May 2 fight to conduct an unannounced drug test and discovered that he had been given an IV for rehydration purposes.
Late exemptio frp, USADA
While the substances in the IV were not banned by WADA, the fact that they were given intravenously was not permitted, the report said, and Mayweather was only given a retroactive therapeutic use exemption (TUE) by USADA 19 days later.
"We believe it is important to immediately correct the record regarding the false suggestion that Floyd Mayweather violated the rules by receiving an IV infusion of saline and vitamins," USADA said in a statement on Thursday.
"As was already publicly reported in May of this year by the Nevada State Athletic Commission (NSAC), Mr. Mayweather applied for and was granted a TUE by USADA for an IV infusion of saline and vitamins that was administered prior to his May 2 fight.
"Mr. Mayweather's use of the IV was not prohibited under the NSAC rules at that time and would not be a violation of the NSAC rules today."
According to USADA, both the NSAC and Team Pacquiao were notified about the TUE after it was granted, "even though the practice is not prohibited under NSAC rules."
For the NSAC, the main issue here is creating a closer working relationship with USADA.
"In the state of Nevada, we are the only entity that is allowed to approve therapeutic use exemptions for any and all combatants," NSAC executive director Bob Bennett told Reuters.
"It's incumbent upon USADA and the Nevada State Athletic Commission to work hand in glove in an effort to combat performance-enhancing drugs in boxing, mixed martial arts, etc.
"Once we can have a better understanding of what USADA's position is on TUEs and ours, this media controversy will end. It adds no negative narrative on Floyd whatsoever. He has set the standard for all fighters [in combatting doping]."
Mayweather will put his unbeaten record on the line when he defends his WBC and WBA welterweight titles against fellow American Berto in Las Vegas at the MGM Grand Garden Arena on Saturday.