Tarrare was slim and of average height. He was described as having unusually soft fair hair, and an abnormally wide mouth in which his teeth were heavily stained, and on which the lips were almost invisible. When he had not eaten his skin would hang so loosely that he could wrap the fold of skin from his abdomen around his waist, and when full his abdomen would distend "like a huge balloon". The skin of his cheeks was wrinkled and hung loosely, and when stretched out he was able to hold twelve eggs or apples in his mouth. His body was hot to the touch and he sweated heavily and constantly suffered from foul body odour; he was described as "stinking to such a degree that he could not be endured at twenty paces". This smell would get noticeably worse after he had eaten, his eyes and cheeks would become bloodshot, a visible vapour would rise from his body, and he would become lethargic, during which time he would belch noisily and his jaws would make swallowing motions. He suffered from chronic diarrhoea, which was said to be "fetid beyond all conception". Despite his large intake of food he did not appear either to vomit excessively or to gain weight.
A meal had been prepared for 15 labourers near the hospital gates; although generally hospital staff restrained Tarrare in the presence of food, on this occasion Courville allowed him to reach the table undisturbed. Tarrare ate the entire meal of two large meat pies, plates of grease and salt and four gallons of milk, and then immediately fell asleep; [...] On another occasion Tarrare was presented with a live cat. He tore the cat's abdomen open with his teeth and drank its blood, and proceeded to eat the entire cat aside from its bones, before vomiting up its fur and skin. Following this, hospital staff offered Tarrare a variety of other animals including snakes, lizards and puppies, all of which were eaten; he also swallowed an entire eel without chewing, having first crushed its head with his teeth. [...]
Efforts to keep him on any kind of controlled diet failed; he would sneak out of the hospital to scavenge for offal outside butchers' shops and to fight stray dogs for carrion in gutters, alleys and rubbish heaps. He was also caught several times within the hospital drinking from patients undergoing bloodletting, and attempting to eat the bodies in the hospital mortuary. Other doctors believed that Tarrare was mentally ill and pressed for him to be transferred to a lunatic asylum, but Percy was keen to continue his experiments and Tarrare remained in the military hospital.
After some time, a 14-month old child disappeared from the hospital, and Tarrare was immediately suspected. Percy was unable or unwilling to defend him, and the hospital staff chased Tarrare from the hospital, to which he never returned.