Happiness 2 by Shuzo Oshimi

Makoto Okazaki is changing. Having turned his former bully Yuuki into a strange kind of friend, he’s been forced to help confront Yuuki’s own tormentors. But even if Makoto’s bizarre new physiology might solve one problem, it is creating others. And behind all of Makoto’s problems lurk horrifying questions: Who was the girl who attacked him? What is happening to his body?

Why is he still so thirsty?