Alexander, 24, was reinstated on Monday after being suspended for the opening four matches of the season.
But a second violation has led to a 10-game ban without pay, ruling him out until the Panthers' December 21 clash against the Cleveland Browns.
In a statement released by the NFL Players Association on Alexander's behalf, he apologised and said it was a mistake he made "months ago".
“I'm very sorry to the Panthers organisation, my team-mates, and Panthers fans that my mistakes from many months ago will prevent me from contributing for several more weeks," Alexander said.
"This discipline arose from a violation that occurred many months ago. Since that violation, I have continued to grow, and I will continue to work hard, as I have been doing, to stay in shape and be a major contributor upon my return.
"Through God's favour, I will continue to show through my behaviour that I have learned some valuable life lessons."
The Panthers hold a 2-2 win-loss record and sit second in the NFC South this season.