Ichihashi, 30, who is believed not to have eaten anything since his arrest on Nov. 10, lost his balance when he was heading to the room from his cell, prompting the police to give him the injection, the lawyer said after meeting with him.
Ichihashi was quoted as telling the lawyer that he did not want the shot.
He has been arrested on a technical charge of abandoning the body of Hawker, 22, in a sand-filled bathtub in his apartment in Ichikawa, Chiba Prefecture, in March 2007.