Dallas owner Jerry Jones acknowledged the ban to reporters Monday night, saying the 27-year-old Scandrick failed a test.
The league confirmed Scandrick violated the league's policy on performance-enhancing substances. The violation reportedly occurred while he was vacationing in Mexico.
Scandrick started 15 of 16 games for the Cowboys last season, making 64 tackles and grabbing two interceptions.
Scandrick will be eligible to return to the Cowboys on September 29. The league is allowing him to play in all the team's exhibition games.