The Demons could be real because.............. ....
Im sorry if this has been mentioned before, but the main girl is given the pictures by the asian girl which have a drawn depicition of the demons, so I guess its possilbe she merely imagined them, although it begs the question if it was a drug why did it all of sudden work at that time , seemingly to Mike and the main girl at the same time.
However, lest we not forget the bald guy who gets killed with his pregnant wife, saw a similar looking demon, even if a drug caused karens delusions, how the hell would he see the same thing a drug cannot create such a similar delusion to two people.