Calvin Johnson retired very early in his NFL career. 

He was one of the best to ever quit at the top of his game.  

Now he’s explaining why he decided to hang it up after only ten years.  

RELATED: Calvin Johnson Getting into the Marijuana Dispensary Business

Maybe it had something to do with all the concussions? 

Megatron says he had at least one a season. 

Johnson also talked about smoking weed after each game. 

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