Memphis Grizzlies guard Nick Calathes will be suspended 20 games for testing positive for a banned substance and violating NBA's anti-drug policy, league sources told Yahoo Sports.

The banned substance – Tamoxifen – was part of an athletic supplement that Calathes had been using, sources said.

NBA lab results found no traces of synthetic testosterone or performance-enhancing drugs, and, as one source told Yahoo Sports, "There was no intent, nor advantage gained here."

Calathes had developed into an important part of the Grizzlies' guard rotation and will now be lost for their first-round Western Conference playoff series against the second-seeded Oklahoma City Thunder. There will be a grievance process for Calathes, but it won't immediately stop the suspension.

The suspension would carry into the 2014-15 season, barring a deep playoff run to the Western Conference finals or NBA Finals.

Tamoxifen is also on the Major League Baseball and International Olympic Committee's banned substance list, because it has been proven to reduce the side effects of steroid use and increase testosterone production. The drug is generally used in treatments for breast cancer in women.