Stephenson was listed as the first-team right tackle on the team's training camp depth chart.
In a statement, Stephenson said he did not check with the league before using a medication.
"I'm extremely sorry that I failed to check with the NFL, NFLPA or Chiefs medical team before I took a medication that requires a therapeutic use exemption. The rules are strict and without a therapeutic use exemption, the consequence is a four-game suspension...
It is not a mistake I will ever make again."
The Chiefs also released a statement on Stephenson's suspension.
This morning the NFL notified the team of Donald Stephenson four-game suspension. Obviously losing Donald is disappointing, but we are in full compliance with the league’s policy. We will have no further comment on the situation.
Stephenson will be eligible to return for Kansas City's Week 5 game against the San Francisco 49ers.
- Paul Palladino