Jackson can have his case reviewed no earlier than Aug. 31, 2013. It is his third drug suspension since the Tampa Bay Buccaneers drafted him in 2007.
Jackson, 27, was released by the Bucs early in this offseason when he failed a physical. A few days later, he signed with the Redskins, whose defensive backs coach is former Tampa Bay head coach Raheem Morris.
Jackson played well for the Redskins and seemed poised to win a job as their starting free safety.
His suspension leaves the Redskins with Brandon Meriweather and Madieu Williams as the likely starters at safety, with DeJon Gomes, Reed Doughty and Jordan Bernstine as the backups pending Friday's final cuts.
Jackson was suspended for four games in 2009. He missed 14 games in 2010 and six in '11 when he was suspended again.
Information from The Associated Press was used in this report.