The league announced the suspension will begin immediately even though Moffitt is on injured reserve and out for the season with a right knee injury. He will not be paid for the final four weeks of the season.
Moffitt's agent, Michael George, said that Moffitt's suspension was due to his use of a prescription drug used to treat hyperactivity. George said that Adderall -- the drug that Moffitt used -- is banned under the league's policy on performance-enhancing substances, unless a player has pre-approval from the league. Moffitt lacked that league approval, resulting in the suspension.
George said there are no plans to appeal the suspension because of the strict nature of the league's policy. George said any appeal could potentially carry the suspension over into next season and that it was best to serve the suspension now since Moffitt was already done for the year because of the knee injury. Moffitt underwent surgery to repair the ligaments just before Thanksgiving.
George emphasized the suspension was not for a substance like steroids.
"(The Seahawks) know that it wasn't for anything like people would be thinking," George said.
Moffitt was Seattle's third-round pick in last April's draft out of Wisconsin. He started the first nine games at right guard before suffering his injury in Seattle's Week 10 win over Baltimore. He's one of three starting offensive linemen currently on injured reserve for the Seahawks, along with right tackle James Carpenter and left tackle Russell Okung.
The Seahawks placed Okung on injured reserve on Tuesday after he suffered a torn pectoral muscle in last Thursday's 31-14 win over Philadelphia. The Seahawks signed guard Mike Gibson to add depth on the offensive line, but the big changes are what's happening with the starters.
Paul McQuistan will move from right guard to left tackle to take over for Okung, coach Pete Carroll said. Lemuel Jeanpierre, who started at center earlier this season against Cleveland, will move in at right guard.
Information from The Associated Press contributed to this report.