Toni's former team-mate at Fiorentina twice tested positive last month for anti-obesity drug sibutramine.
Mutu is temporarily suspended and could face a lengthy ban.
"It is inexplicable," said Toni to Italian newspaper Il Messaggero."It's incredible.
"He has no reason to lose weight and I ask myself how come he took that drug.
"It's stupid to throw away everything just like that.
"We know there are anti-doping controls done and that we cannot take anything that is banned.
"It's like going at 150km against a wall, you may escape unharmed but the likelihood is that you risk getting hurt."
Romania international Mutu served a seven-month ban five years ago and was sacked by Chelsea after testing positive for cocaine.