Free-agent quarterback Mark Sanchez has been suspended for the first four games of the 2018 season for violating the NFL's performance-enhancing drug policy, the league announced.

Sanchez acknowledged in an Instagram post Friday that he had tested positive for a banned substance.

"I was blind-sided by the news and I want to say unequivocally that I have never cheated or attempted to gain a competitive advantage by using a banned performance enhancing substance," he wrote.

Sanchez blamed the positive test on an "unknowing supplement contamination." He said he had clean tests 30 days before and 12 days after his positive test.

Even so, Sanchez said he accepts the suspension and will "look forward to getting back onto the field this season."

Mark Sanchez says his PED suspension from the NFL was the result of a supplement contamination. Quinn Harris/Icon Sportswire

Serving as the No. 3 quarterback for all of 2017, Sanchez was a mentor for Chicago Bears rookie Mitchell Trubisky, the No. 2 overall pick, while not throwing a pass during the regular season.

Sanchez, the fifth overall choice of the 2009 NFL draft, started 72 games for the New York Jets and Philadelphia Eagles from 2009 to 2015 before taking on backup roles with the Dallas Cowboys (2016) and Bears.

He said in the Instagram statement that he has previously passed 72 drug tests and that his supplement regimen has been consistent for five years.

ESPN's Jeff Dickerson contributed to this report.