Patrick tested positive for the drug Adderall, an amphetamine, his agent Kevin Omell said on Monday. Adderall is regularly prescribed to treat attention deficit hyperactivity disorder.
"He literally took one pill one time driving back from Flagstaff to keep him awake," Omell said, according to Scout.com.
The agent says Patrick took the drug during the team's bye week in mid-October.
According to Omell, Patrick got the pill from a friend who had used the drug to stay up late studying.
"It was a completely honest mistake and he's really remorseful," Omell said.
A seventh-round draft pick out of Delaware in 2007, Patrick caught a touchdown pass in Arizona's Super Bowl loss to Pittsburgh.
He caught 11 passes for 104 yards in the regular season last year and was expected to compete for the starting job in training camp.
Patrick will be eligible to return to the team for its Oct. 18 game at Seattle.
The test result was first reported by Scout.com.
Information from The Associated Press was used in this report.