“ Psychedelics act on certain kinds of neurotransmitters called neuromodulators and literally disintegrate your belief-updating, freeing this neural population from the evidence and the influences of this population. You’ll now be unable to call on high-level representations of selfhood to constrain your qualitative experience at a lower level of abstraction. You may experience a lower-level disillusion of the ego, literally the “self” representation may now not be able to influence experience. You might be attending to vivid sensations that would now have other interpretations that are not glued together by a self-centered, egocentric narrative. You will have alternative explanations for what caused this sensation and what caused that sensation. All of this is perfectly sensible under a generative model where we remove the constraints of one level of processing from another. If that’s going on, you now have the hypothesis that if I am me, there’s another hypothesis, I’m not me. If you start to remove the evidence of these two hypotheses, then you create an enormous uncertainty about me-hood. You’ll get depersonalization and possibly a bad trip.”

I’ve done some so-called heroic doses that dissolved my ego, what he calls depersonalization. If one is not prepared for that sort of thing to happen and/or doesn’t have the appropriate setting or people around to make sure you don’t hurt yourself or others then yeah it can certainly turn into a bad trip.