It seems a lot of people in the comments are a bit confused, so I will try explain a bit.
Wizards can not use their powers as they please in this world, because using them drives them insane. Thus a purifier is needed to cleanse them through touch. However, in the novel, the protagonist, Zahid, is able to prevent himself from going mad by consuming the purifier, Ertha, in the beginning of the novel.
(Yes as in cannibalism)