Jenkins released a statement saying he was "shocked and confused" to learn that he had tested positive for a banned performance enhancing substance.
"It's an obscure substance that I've never even heard of, and I still don't know how it got into my body," the statement said. "My only guess is that it came from one of the supplements I was taking around the time of the test, even though none of them listed anything banned."
Jenkins remains eligible to practice at training camp and play in preseason games. He was on the field working with the first-string unit during the afternoon practice when the suspension was announced.
Jenkins will be eligible to return to the active roster on Sept. 30 following the team's Week 4 game against the Oakland Raiders.
A second-round pick in 2011, Jenkins missed his rookie season with a right knee injury. He returned last year to start 14 games.
Jenkins is the eighth Redskins player to receive a drugs-related suspension in less than three seasons. Left tackle Trent Williams, tight end Fred Davis and cornerback Phillip Buchanon served four-game sanctions in 2011. Cornerback Cedric Griffin and tackle Jordan Black were docked four games last year.
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