Slimani accepted the charge but claimed that the suspension was excessive - this was later denied by the independent regulatory body.
It means that Slimani will miss the last three games of the season and has probably played his last in a Newcastle shirt, unless they make the loan move permanent.
"Islam Slimani will miss Newcastle United's next three matches after he was charged with violent conduct," the FA said in a statement.
"He accepted the charge but argued the suspension was excessive. This was rejected by an independent regulatory commission.
"The forward was involved in an off-the-ball incident during Saturday's game against West Bromwich Albion."