The NFL has suspended Jo-Lonn Dunbar for breaching the league's policy on performance-enhancing substances.
The St Louis Rams linebacker will miss the opening four games of the 2013 season.
The 28-year-old told The AP: "I just want to say that I did not take a performance-enhancing drug. I mistakenly took something that prompted a failed drug test.
"I sincerely apologise to my teammates [and] the organisation. I am deeply saddened by this. But I plan to serve my four-game suspension and get back to playing football as soon as possible."
Dunbar made 115 tackles, 4.5 sacks and two interceptions for the Rams in 2012.
