Chiefs' Stephenson banned

Kansas City Chiefs' starting right tackle Donald Stephenson has been banned by the NFL for four games following a performance-enhancing substance violation.

The 25-year-old has not missed a Chiefs game since being taken in the third round of the 2012 NFL Draft and blamed his lack of a theraputic-use exemption for the violation.

"I'm extremely sorry that I failed to check with the NFL, NFL Players Association or Chiefs medical team before I took a medication that requires a therapeutic use exemption," Stephenson said in an apology statement released by the players' union.

"It is not a mistake I will ever make again. I will train extremely hard during my time away from the team and I will be ready to contribute to my team's effort to win a Super Bowl as soon as I return."

Stephenson will miss the clashes with Tennessee, Denver, Miami and New England.

A statement released by the Chiefs read: "Obviously losing Donald is disappointing but we are in full compliance with the league's policy."