Much has been written about Monk's supposed mental illness but it is contested whether he had one or not. The following article is a good one on the subject: The Definitive Monk Bio: So, Was He Crazy, or What?
Dr. Schlesinger writes: "The fact is that, like many laypeople (as well as far too many professionals), Kelley confuses a serious condition with an eccentric personality. Before psychiatry made everything a [reimbur$able] flaw, "cyclothymic" people were simply considered "moody"; they didn't suffer from any inherent mental disorder. When people experienced negative life events, they were able to panic, rant, rail at the gods, weep, and even act bizarrely for a bit without incurring a dark diagnosis that subsequently followed them to the grave (and beyond)....
Moreover, for years Monk took "vitamin shots" which were laced with Benzedrine (speed) (pp. 267, 318). This could agitate anyone, causing addiction and other life disruptions, and the doctor who peddled them eventually had his license suspended. But even when Monk refused to play or leave his room, his mind reportedly remained as sharp and witty as ever (p. 444). This is not psychosis: this is choice. His recurring ability to bounce back from bad behavior to disciplined, brilliant music-making (sometimes the next day) is a testament to his strength, not his craziness".
She concludes "Yeah, Monk was crazy, all right: crazy like a fox."