The 31-year-old tested positive for the banned stimulant ephedrine after last season's Championship playoff semi-final second leg against Preston North End.
United suspended Kenny almost immediately and the FA initially banned him from all football on July 22.
Kenny said the substance was in his system but that it had come from an over-the-counter product to ease congestion and had not been intended to enhance sporting performance.
The FA backdated the ban to 22 July, leaving him free to return to action in April.
"Whilst we found that the player satisfied us on the balance of probabilities that the substance was not taken with the intention of enhancing sporting performance, his admitted conduct displayed significant fault," regulatory commission chairman Christopher Quinlan said in a statement.
"A professional sportsman has a strict responsibility to ensure prohibited substances do not enter his/her body. In this instance Mr Kenny knowingly ingested an over-the-counter medicine above the prescribed dosage without reading the accompanying package or leaflet and without reference to his club's doctor or other medical staff."
Sheffield United manager Kevin Blackwell said he thought the length of the ban was "unbelievable."
"The Commission accepted he didn't take the drug for performance-enhancing... so we're a little surprised he's been done for nine months," he told the BBC.
"I don't understand the length of the ban. It's unbelievable.
"Paddy's devastated. At no time did he think it would come to this - it's a major lesson to every sportsman or woman out there."
