The Mirror and the Drug Dont Prove Anything - SPOILERS
Misty Mirrors
Some people are saying that Anna's breath on the mirrors proves that she is alive.
But it doesnt prove anything.
If someone else (one of the cops, her mother, her boyfriend before he got killed) had seen the mirror mist up, then that would be a massive clue that she was alive.
But it can just as easily be said that she is "obviously alive" because she talks to the undertaker, she unlocks the door, she walks round with a knife, she trashes his office, she is seen by the kid.
However, all of those scenes are were either:
i) only Anna is present
ii) only Anna and the undertaker are present, or
iii) the kid is the only other witness
If the only characters that can see dead people are the kid and the undertaker, then everything we see in the movie is consistent with Anna being dead.
The breath on the mirror does not give us any additional info. EITHER, the undertaker cannot see dead people, in which case the proof that she is alive is the fact that he spends most of the movie interacting with her, and she trashes his office. OR he can see dead people, and the fact that he "sees" her breath on the mirror is no different to the fact he "sees" her do lots of other stuff, and "hears" her talk.
After all, if Dead Anna doesnt "breathe" then she can't "talk" either, because "talking" involves air being forced over the vocal cords.
Something about the gift the kid and the undertaker both have allows them to see and hear the dead, when other people can't. So other people wouldnt see or hear Anna breathe, and would not see or hear Anna talk.
The evidence that the kid also has the gift/curse is that he is able to see his mother move, despite the fact she is apparently dead.
Plausible Paralysis
So what about the fact that Eliot was injecting Anna with "Hydronium Bromide". This is claimed to be proof that she was alive, because a cop had claimed that it can produce a "fake death" of a living person.
This drug doesnt exist in real life.
So, in the fictional world of the movie, it is possible it does exactly what the cop says.
HOWEVER, is that cop a trustworthy character? Maybe he saw a vial of "Hydronium Bromide" at the funeral parlour, and wanted to make trouble for the undertaker by encouraging the boyfriend to believe the undertaker was holding the girl captive.
Alternatively, maybe the cop has read some sort of conspiracy theory on the internet about "Hydronium Bromide", but that doesnt make it true.
At the end of the day, this cop is a bit of a freak who liked playing with corpses. He is a border line psychopath, and nothing he says should be relied upon by anyone. Maybe he does not like to think of himself as a necrophiliac. As part of his fantasy, he tells him that the corpses which he molests are really alive, and are simply in a drug-induced coma.
Apart from this one cop's claims, there is no evidence, in either the real world, or in the rest of the movie, that there are drugs that can make someone appear dead throughout the various examinations by family and friends.