Ah, the Medicine of Youth (ああ、青春の漢方丸, Aa, seishun no kanpōgan) is episode 186 of the Naruto: Shippūden anime.
Story setting of this episode is around Lee's surgery. Naruto, Sakura and Ino are sent in Jofuku Forest to deliver a message to Furofuki. On the way there, they were petrified because an old lady from the village's inn said that the place was haunted. Naruto managed to take a sample of Jofuku flower for Lee's recovery. They also fought a deity representing Gaara, but Naruto managed to defeat him. They arrived at Furofuki's house and completed their mission.
Though the episode takes place before the Sasuke Retrieval Arc, and the apparition of Gaara is wearing the same clothes he wore during that arc. This creates an inconsistency as the apparition is supposed to be brought on by Naruto's team's feelings and at that point in time, they had only seen his original outfit.
This episode has many similarities with the episode 35 "Desperate Journey! Open Dragon's Eyes" of Saint Seiya anime series, it's a filler story and adds a magical medicine retrieved by friends to the manga explanation of the later healing. In the Saint Seiya episode Pegasus Seiya retrives the Water od Life to heal Dragon Shiryu's blindness.