Mark Sanchez has received a four-game suspension for violating the NFL's performance-enhancing drug policy.
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.
Sanchez was the No. 3 quarterback for the Bears last season.