Saul 'Canelo' Alvarez and Gennady Golovkin's May 5 rematch was certainly one of the most anticipated bouts in middleweight history.

The two superstar fighters finally met in the four-cornered ring in September, but fans were left disappointed as the fight resulted in a controversial draw.

Most boxing fans believed Golovkin was a deserving winner, with the big-punching Kazakh dominating his Mexican opponent for most of the rounds.

The big controversial point from that fight was judge Adalaide Byrd's score card, who amazingly had Canelo up 118-110, even handing the last five rounds to him.

However, the controversy between the pair has taken on a whole new level ahead of their second meeting, with Canelo embroiled in a doping scandal.

The Mexican fighter has blamed contaminated meat for the positive clenbuterol test in February and the authorities seem reluctant to make a serious case of it.

And now, Golovkin - who is not usually one for trash talk - has finally snapped, launching a scathing attack on his upcoming opponent and his Golden Boy promoter, Oscar De La Hoya.

"Canelo's team are using these drugs and everybody's trying to pretend it's not happening," Golovkin said.

"This guy, he knows. This is not his first day in boxing. He proves he gets benefits from everyone and he can get away with it. The commentators, commission, doping commission—this is a very bad business, [it's] not sport. Check him on a lie detector and then we can find out everything."

GGG also went one step further by claiming he knew Canelo cheated for the first fight. He doesn't skip on the detail either.

"Before the first fight, I knew he was not clean," he said. "Have a look at the video. He used some pills. It was pretty obvious when his muscles were all with traces of injections. I've known it for a long time."

Wow. Golden Boy president Eric Gomez quickly released a statement after Golovkin's comments.

"It is ridiculous," he said. "[GGG] doesn't sound like a confident fighter. I guess he has a lot of insecurities. There is an investigation [into the clenbuterol] and we are cooperating. When it is complete, all the facts will come out. Canelo is a clean fighter; the facts and statistics back it up."

Despite Golvokin's attack, he has still not asked for the fight to be called off.