Hopkins' brand manager, Doug Sanders, wrote in a text message to ESPN's Adam Schefter that a test in November returned "trace elements of a banned substance" and that tests in both the month before and after were negative.
"I was confused and shocked," Hopkins tweeted later. "I am very mindful of what I put in my body and have always taken a holistic approach, so I am working with my team to investigate how this could've happened.
"But even as careful as I have been, clearly I wasn't careful enough. For that, I apologize to Cardinals fans, my teammates, and the entire Cardinals organization. I never want to let my team down. I fully intend to get to the bottom of this. As soon as I have more information I will share it."
Hopkins will be allowed to play during the preseason and practice with the team until the beginning of the 2022 regular season.
Hopkins, a three-time first-team All-Pro and five-time Pro Bowl selection, will turn 30 on June 6 and is coming off an injury-riddled season that led to the lowest output of his career.
He missed three games in the middle of last season with a hamstring injury and the final four games of the regular season, plus Arizona's wild-card loss to the Los Angeles Rams, after suffering an MCL injury in Week 14 on Monday Night Football. The knee was surgically repaired in December. In February, while making interview rounds at the Super Bowl in Los Angeles, Hopkins said on SiriusXM NFL Radio that his rehab was going great and he was ahead of schedule.
Hopkins finished with 572 yards and eight touchdowns on 42 catches a year after having 1,407 yards. Arizona went 3-4 without Hopkins during the regular season and was then blown out in the playoffs, losing 34-11 to the Rams.
Before his injuries, Hopkins was as good in 2021 as he has been in the past, or better. He had a higher catch percentage, yards per target and yards after catch per reception through Week 8. He was down slightly in yards per route, from 2.1 in his first eight seasons to 2.0 last season.
In two seasons with the Cardinals since they acquired him from the Houston Texans in 2020, Hopkins has 157 receptions for 1,979 yards and 14 touchdown catches.
Hopkins missed just two games in the first eight seasons of his career before missing seven last season.