Warning: long post
TBH I rarely use the word hallucinogenic, I much prefer the word psychedelic. But it's a complex topic filled with semantics and everyone seems to have their own definition.
Opium and other opiods have been described as having visionary/psychedelic properties, and dopaminergic stimulants can make some people see/hear/feel things that aren't there. There are plenty of other ambiguous examples as well.
So, should opium and meth be classified as hallucinogens too? Or even as psychedelics?
I don't think so, otherwise one might say that most drugs are capable of producing psychedelic effects in the right (or wrong) set&setting. One might even say that even music and food are psychedelics, and they might be, but then it justs gets too confusing.
I'd just classify THC and weed as a cannabinoid, thats good enough for me, maybe even as a psychedelic cannabinoid depending on the context. But it shouldn't be confused with serotonergic psychedelics (which are "true" psychedelics), they're not the same.
Same goes for dissociatives, they just don't provide the same type of experience, despite having strong psychedelic qualities (specially at high doses, with your eyes closed).
And btw, none of them are intrinsically better or worse than the others, they all have different qualities.
Regarding hallucinogens, it's a pretty messy subject.
For the sake of simplification, we may say there are 2 main types of hallucinations :
- Simple hallucinations are more like distortions, they usually manifest as an altered perception of color, brightness, shapes, sounds, etc... I personally don't really see them as true hallucinations, even though they technically are.
They can actually have a high degree of complexity, for instance fractals can rearrange themselves into really intricate patterns even to the point of appearing as fractalized entities or beautiful landscapes. In extreme cases they may lay at the limit between the 2 main types of hallucinations.
- Complex hallucinations, those are very different, they tend to make you perceive things that aren't really there and usually carry a much higher level of detail : seeing what appear to be real people/beings/objects, hearing clear voices which don't identify as your own, feeling bugs crawling under your skin and stuff like that (often almost indestinguable from sober reality by the observer).
To me a true hallucinogenic drug would be something capable of producing complex open-eyed hallucinations. I probably wouldn't incluide CEV in this description, but I'd take into account auditory hallucinations (though those are often related to what we call psychosis).
Deliriants would of course be the best example of true hallucinogens, those are mostly anticholinergic drugs. But then again, there are some weird ones like muscimol from amanita muscaria which is primarily a gabaergic, yet it can apparently make you hallucinate as well (I've never tried that one though).
In the end I probably wouldn't classify most serotonergic psychs as true hallucinogens either. Although things get weird at very high doses... So let's say that I haven't made up my mind yet.