Floyd Mayweather did not follow the rules correctly when seeking approval for an intravenous drip before his victory over Manny Pacquiao, the Nevada State Athletic Commission (NSAC) says.
The US Anti-Doping Agency (Usada) gave retrospective permission after Mayweather had the drip of saline and vitamins on the eve of his 2 May fight.
But the NSAC says only it could approve the drip, and not Usada.
Pacquiao wants a rematch, according to the AFP news agency.
Usada said it granted unbeaten American Mayweather, 38, a Therapeutic Use Exemption (TUE) for the infusion, in accordance with NSAC regulations.
But NSAC executive director Bob Bennett told BBC Sport: "Anyone should know worldwide, if you fight in the state of Nevada we are the sole authority for approving a TUE.
"Usada has been known historically as the premier performance-enhancing drugs-testing organisation and they have an outstanding record. But they do not have the authority to authorise a TUE to a fighter in the state of Nevada."
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Why would Mayweather's use of the drip be a problem?
Mayweather's drip, administered to combat dehydration after a tough training session, had already attracted controversy.
A report claimed he received favourable treatment from Usada in being given permission to have it. It said, although the substances in question were not banned, an IV drip "can dilute or mask the presence of another substance that is already in the recipient's system".
Usada said the report was "riddled with significant inaccuracies and misrepresentations", while Mayweather highlighted his own anti-doping stance.
"I follow and have always followed the rules of Nevada and Usada, the gold standard of drug-testing," he said.
"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."
What should Mayweather have done?
Bennett explained the process of successfully obtaining a TUE, adding that it was very rare for one to be granted so close to a fight.
"The process is you go online and you submit an application with supportive evidence from your doctor. Upon our receiving it, we give the application to our doctor. After his review we will determine if a TUE is approved.
"We are very meticulous, we look a each TUE very thoroughly before our doctors make a final decision. We want to do all this for the health and safety of the fighter.
"I don't recall a time when we have authorised a pre-fight TUE for an intravenous drip. I'm not saying it hasn't ever happened, but I don't recall it."
Will there be a rematch?
BBC Sport's Ade Adedoyin in Las Vegas: "The Manny Pacquiao camp are quite unhappy about his situation.
"They point to the fact that before Pacquiao went out to face Mayweather he asked for permission to have an injection of painkillers but was denied by the NSAC.
"One of his business advisors has spoken to the media and has been saying how ironic it is for Mayweather to be in this situation now.
"He has also says that perhaps the best way forward is for Mayweather and Pacquiao to have a rematch."
What does Pacquiao think?
Pacquiao has been addressing the media in his native Phillipines and has called for a rematch.
The 36-year-old was refused an anti-inflammatory shoulder injection before his fight with Mayweather - the richest in the history of the sport - after failing to notify the correct authorities.
He later blamed the injury for his points defeat, claiming it stopped him using his right hand.
"That is why I want a rematch," AFP news agency quoted him as saying. "One without any injury and with fair play. No favouritism. Not one where the Mayweather camp gets to dictate all the terms and conditions.
"The Mayweather camp accused me of using performance-enhancing drugs. Now look what happened. The truth has finally came out and I was vindicated.
"If needed, the NSAC should impose the appropriate sanction to sustain its credibility and to show the world they did not give preferential treatment to the Mayweather camp."