I'm rewriting my old post to make it more logical.
I think David might have schizotypal personality disorder; while many aspects could be mistaken for the effects of phencyclidine-induced "schizophrenia," his delusions of persecution*³, of connections*², and so on are personally linked to him, if he didn't have a personality disorder, the whole game would be just a mess of psychotic symptoms. He can't have schizophrenia because his psychosis then will continued after detox, and in our case, David wakes up without psychosis.
(If anything, I'll focus more on the most logical ending)
So, reasons:
1. David is quiet —
Andreas described him as a mysterious and quiet man, which is exactly what he is in the game;
2*. Idea of connections (I don't know the correct translation) —
As I said, everything is connected to David in one way or another, unlike the case if he weren't schizotypal;
Detailed description:
In David's mind we are shown that the cottage in the woods is supposedly connected in some way to his Addiction;
I’m not sure if this counts, but he has very strange mental associations regarding levers and doors and the way things shift as he moves back and forth throughout the game (it’s hard to describe precisely). I might interpret this as "if I touch something here, something will happen over there" (That’s a vague example). To put it more precisely: a conviction that his thoughts or actions (which aren't the cause) can somehow influence events around him;
* He perceives a threat in absolutely everyone. Of course, this is a bit of a stretch since we don't know all the details, but viewing passersby as enemies could imply that he believes they know something about him or hate him (a combination of paranoia and ideas of connection);
* He also likely dislikes being watched (judging by the drawings of eyes everywhere) because he feels his thoughts are being read (another instance where paranoia and the concept of interconnectedness intertwine, though colored by magical thinking).
3. Paranoia —
He clearly thinks he is being followed (the sign in the disabled toilet at the very beginning, the sign behind the box on the stairs down);
He believes that doctors and passers-by want to hurt him (see q. 2.3*↑)
See q. 2.4*↑
4. Dissociations —
He dissociates from the fact that he is a drug addict, you can read more specifically here;
5. (doubtful) Magical thinking —
If we take the game's title literally, he's afraid of monsters. Incidentally, this could also be interpreted as him believing that humans aren't actually humans, but monsters (also paranoia).

















