The eight-time Pro Bowler will be eligible to return Oct. 20 against the New York Giants.
"Patrick recognizes how disappointing this is for everyone in the organization as well as our fans," the Cardinals said in a statement. "As we all do, he understands that regardless of intent he is ultimately responsible for everything he puts in his body.
"Our coaching staff and our players are now aware that we will have to play the first six games without Patrick and they understand they have to rise to the challenge that it presents."
First-time offenders of the league's PED policy typically receive four-game suspensions, but the NFL can enforce another two games if a diuretic or masking agent is detected in addition to the prohibited substance, per ESPN's Dan Graziano.
Peterson made a shock trade request midway through last season, but later apologized and committed to the Cardinals. However, speculation about the 28-year-old's future continued after an April report said Peterson was angry at someone in Arizona's front office because of a comment that "pissed him off."
He also skipped voluntary workouts in late April, though head coach Kliff Kingsbury dismissed any rift between the team and the All-Pro corner.