Russian heavyweight contender Alexander Povetkin's "B" sample came back positive for the banned substance meldonium on Thursday.

Povetkin's mandatory shot at heavyweight world titleholder Deontay Wilder, scheduled to take place May 21 at the Khodynka Ice Palace in Moscow, was called off after his "A" sample came up positive for meldonium nine days before the fight. The sample had been taken in a random urine test conducted by the Voluntary Anti-Doping Association on April 27.

Povetkin, 36, had the right to have his "B" sample tested, and he did. According to the VADA testing report, which was obtained by ESPN.com, it was also positive for meldonium.

Alexander Povetkin's mandatory shot at heavyweight world titleholder Deontay Wilder was called off after he tested positive for meldonium nine days before the fight. Dmitry Korotayev/Kommersant/Getty Images

The report said that the test was "adverse" and that the "urine specimen contains meldonium."

The WBC said Wednesday that it is conducting "an in-depth investigation and evaluation of the situation" and that a decision on Povetkin is forthcoming.

Meldonium is the drug that tennis star Maria Sharapova and many other athletes recently tested positive for. It is used to increase blood flow and carry more oxygen to muscles and, therefore, is said to enhance stamina, a trait boxers would want in a long fight.

Meldonium was approved to be added to the banned substance list by the World Anti-Doping Agency in September, and the ban went into effect on Jan. 1.

Povetkin admitted to using meldonium but said he last consumed it in September. However, that does not line up with his test results. He was randomly tested by VADA on April 7, 8 and 11, and each test came back negative for any banned substances, according to the test reports also obtained by ESPN.com. But his test on April 27 was positive for meldonium.

Wilder (36-0, 35 KOs), a 2008 U.S. Olympic bronze medalist from Tuscaloosa, Alabama, was supposed to make his fourth title defense against Povetkin (30-1, 22 KOs), a 2004 Olympic gold medalist. He loomed as Wilder's most significant opponent, and the fight was one of the most significant that could be made in the heavyweight division.

But with Povetkin's career in limbo and the fight off, the WBC on Wednesday announced that Wilder, 30, would be permitted to take an optional bout, which his handlers hope to set up for him this summer.