Former boxing champion Lucian Bute said a supplement he took to help him sleep contained the banned substance Ostarine and led to his positive doping test.
Bute intends to take legal action against KorvalLabs of Calfornia, which produced the supplement.
He said Ostarine was not listed among the ingredients and that it may accidentally have got into the bottles.
He said his physiotherpist Angel Heredia specifically ordered a product that did not contain banned substances.
The Romanian-Canadian intends to plead that he never knowing took a banned substance to the Washington Athletic Commission, which is to rule on whether he will be suspended.
Bute tested positive after fighting World Boxing Council super-middleweight champion Badou Jack to a majority draw on April 30 in Washington, D.C. Days later it was announced that his urine A sample tested positive.
Results of the B sample received this week confirmed the positive test.
Bute held the International Boxing Federation super-middleweight title from 2007 to 2012.
He said he intends to continue boxing regardless of what the commission rules in hope of regaining a world title one day.