Ronaldo's weight problem is linked with hypothyroidism.
AC Milan has prescribed drugs to combat the Brazil striker's under-active thyroid, and advised the Italian Olympic Committee and the World Anti-Doping Association, the Gazzetta dello Sport reported Friday.
"You'll see (him) soon, very soon. It will be a surprise for everyone. He looks like a model," Milan coach Carlo Ancelotti said.
Ronaldo was criticized for being overweight during his four and a half seasons with Real Madrid. He transferred to Milan in January.
Ronaldo arrived at a training camp this season weighing 95.5 kilograms (211 pounds). He's currently down to 90 kilograms (198 pounds). The goal is to reach 87-88 kilograms (192-194 pounds) - his weight when he began playing for Inter Milan 10 years ago.
"He's going to have a great season," Ancelotti said. "He's very motivated, he really wants to show that he's still the best striker in the world."
Ronaldo currently has a muscle problem in his left knee and is expected to miss Milan's first few matches this season.
He can play for Milan in the Champions League this season after sitting out the team's victory in the competition last season because he began the season with Madrid.