The super featherweight world champion of the World Boxing Council (WBC), Oscar Valdez, has broken his silence on the drug testing controversy that surrounds his upcoming defense against Olympic gold medal winner Robson Conceicao.

The contest is taking place on September 10 from Tucson, Arizona.

Valdez's defense came under fire earlier this week, when it was revealed that he tested positive for the banned substance phentermine, which is best known as an appetite suppressant.

The substance is banned by the Voluntary Anti-Doping Association (VADA).

But, the substance is not considered prohibited when it is used 'out of competition' by the World Anti-Doping Agency. 

Valdez came up positive for the substance from a test taken on August 13. He came up negative on a test taken on August 30.

After a review of the situation, both the Pascua Yaqui Tribe Athletic Commission and the WBC allowed the fight to go forward.

"I have never used banned substances to improve my performance. I never have and never will. I am respectful of anti-doping tests since my time as an amateur and now as a professional," Valdez said.

"And since I've been a professional I have undergone more than 30 anti-doping tests, which is something we always ask for and we make sure that [drug testing] is in the contract, to do the VADA tests to keep the sport clean and to make sure that no one has any advantage."

WBC President Mauricio Sulaiman explained his organization's decision to allow Valdez's defense to move forward.

"We formed a panel of experts, we had meetings for three days. We had meetings and an audience with Valdez and his team, with the boxing commission, which is the jurisdiction where the fight takes place. This substance does not give you any competitive advantage, this substance it is the equivalent of having three energy drinks," Sulaiman said.

"During the investigation with Valdez no flaw was detected, no intention, no situation in which he would have wanted to take advantage and this substance does not give him any advantage. That is why we decided to proceed with the world title fight."