Lewis, who will miss the rest of the regular season for the NFC North-leading Lions, admitted to the "mistake" in a statement.
"I apologize to my coaches, teammates, the Lions organization and our fans for my actions. I have let my team down and there's nothing worse than that," he said.
"I will not make the same mistake again. I look forward to returning to the team after I serve my suspension."
The 25-year-old, who has mainly played on special teams, tested positive for the drug Adderall, the Detroit Free Press reported.
Lions General Manager Martin Mayhew said while the team is "disappointed that Travis violated League policy and will miss the next four games, we expect that Travis will learn from this experience and not repeat it."
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