Unless allegations against Peyton Manning and his use of performance enhancing drugs (PEDs) are proven beyond a reasonable doubt, I do not think that these accusations will impact nor ruin his career. Manning is a stellar player and players of his caliber often have to endure accusations against them and society is used to this and much worse already. So unless it is proven that he cheated by using PEDs, his career will remain intact.
Peyton Manning should not worry about his career. The accusations against him will only ruin his career if people were to believe them and I think his character alone proves that those accusations are false. Mannings has a long line of devoted supporters who will rally around him and keep him on the field. This will not dim the light that he has brought to the game of football.
I'm not sure the accusations that Manning used illegal substances will ruin his career. I'm not sure the accusations will actually stick to him. I think it will be important to find out all of the details of this before passing any judgement. If he did use them, I don't think they would help him that much.
The accusations that Peyton Manning used illegal substances will be a story for a short time. He has been in professional football for such a long time that people will rally around him. He is a football god to many of the fans, and will be inducted into the hall of fame one day. This is just a bump in the road for the football star.