OKLAHOMA CITY -- Suspended Memphis Grizzlies guard Nick Calathes is mounting a defense against a 20-game suspension handed down Friday for violation of the NBA's drug policy, citing it as unfair. The league, however, plans to stand by its ruling under the collective bargaining agreement.

Calathes was suspended less than 24 hours before the Grizzlies started their playoff series against the Oklahoma City Thunder for use of the drug Tamoxifen, which is one of the 140 substances banned by the NBA's performance-enhancing drug policy. Tamoxifen is not directly a PED but a masking agent for other substances, including synthetic testosterone.

Memphis will be minus Nick Calathes for 20 games after he violated the NBA's anti-drug policy. Kyle Terada/USA TODAY Sports

The National Basketball Players Association has the option of asking for an expedited grievance to be heard within 10 days. It remains unlikely that the process will play out before the end of the playoffs. Unlike in Major League Baseball, appealed suspensions do not result in a stay.

"[It was] an over-the-counter supplement to treat a private but common medical condition; the NBA rejected it because it doesn't require a prescription," attorney David Cornwell, who is representing Calathes, told ESPN.

"Our tests identified tamoxifen in a supplement Nick [used] for a legitimate medical condition and our tests confirmed that Nick did not have testosterone or any other PED in his body. Despite this irrefutable, objective scientific evidence, the NBA's response was, 'Oh well.' This is indefensible because no legitimate purpose is served by suspending a man who the NBA knows was not cheating."

NBA general counsel Rick Buchanan, who oversees the league's drug-testing program, told ESPN the agreement on PED enforcement between the league and the players union does not allow it to consider intent. The players have a strict liability on knowing what they put in their bodies.

"Nutritional supplements that are available over the counter are not regulated by the FDA and there's no regulations of what goes into these supplements. As a result there's a risk of taking them," Buchanan said. "At the start of the season the league and the union send a joint letter to the players about the dangers of taking supplements and it tells them there's no guarantee of label integrity."

The Grizzlies, who were not notified officially of Calathes' impending suspension until this week, were stunned.

"We've had a lot of adversity all season long and now we've got to step up our games," Grizzlies coach Dave Joerger said Saturday. "It's an NBA deal and that's where we have to leave it."

Players are tested up to six times a year randomly, four times during the season and twice in the offseason. Samples are separated into an "A" and "B." If the "A" sample comes back positive, the player is given the option of having the "B" sample tested by a different lab. If both samples test positive, the player meets with an approved doctor to examine if there is a legitimate alternative medical explanation.

Cornwell feels Calathes had a legitimate medical explanation. Buchanan said he cannot talk about specific cases because of the league's confidentiality requirements. However, the fact that Calathes was suspended makes the doctor's recommendation clear.

"This player is not a cheater but unfortunately our program treats him as if he is one," acting players union executive director Ron Klempner said. "It's a failure of our policy and it's something that we plan on addressing when we next discuss our policy, which I hope and expect is before our agreement expires."

Calathes is just the fifth player to be suspended by the NBA for PEDs since 2007 and the first since O.J. Mayo was banned in 2011. Calathes is the first player to receive a 20-game ban; the suspensions were increased from 10 games as part of the CBA signed in late 2011.

The league and the players have talked but not yet agreed to expand drug testing to blood tests or to check for HGH, which is banned by the NBA but not currently tested for. The World Anti-Doping Association is still working to set testing standards for HGH and the players' union currently has no permanent executive director, which has halted talks to amend the anti-drug program.

Calathes, a 25-year-old rookie who spent the last four years playing overseas, is primarily Mike Conley's backup at point guard. He averaged 4.9 points and 2.9 assists for the Grizzlies this season. Beno Udrih, who was acquired last month, will likely get Calathes' minutes against the Thunder.

Information from ESPN The Magazine's Chris Broussard was used in this report.