According to this article, agonism of the 5-HT$_{2A}$ is necessary for hallucinogenesis. Now, not all hallucinogens are primarily serotonergic, but does there exist hallucinogens that aren't at all serotonergic? If so, that means there are hallucinogens that don't agonise the necessary receptors for hallucinogenesis.

Would that mean that these non-serotonergic hallucinogens only produce visual distortions (illusions), and not "proper" hallucinations? And do all serotonergic hallucinogens activate the 5-HT$_{2A}$ receptors, or are there serotonergic hallucinogens that don't activate exactly this receptor, but still primarily or partially operates on all the other serotonergic receptors?

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    $\begingroup$ The article does not say that 5-HT2a agonism is necessary for hallucinogenesis; it says that it's necessary for this class of drugs. The evidence for this is that if you use an antagonist at 5-HT2a receptors you block the psychedelic effect when administered with these drugs, and that similar molecules in the same class that have little or no activity at the 2a receptor are not psychedelic. You asked a very similar question already here: psychology.stackexchange.com/questions/26710 and got answers about ketamine and PCP. $\endgroup$
    – Bryan Krause
    Feb 27 '21 at 19:55
  • $\begingroup$ @BryanKrause So 5-HT2a agonism is only necessary for hallucinogens to produce hallucinations, but drugs within another class may produce hallucinations without 5-HT2a agonism? Also, would it be correct to say that hallucinogens are drugs that induce hallucinations and/or visual distortions (illusions), whereas serotonergic hallucinogens are drugs that necessarily produces hallucinations (and potentially perceptual distortions). Psychedelics can be used as a synonym for hallucinogens, serotonergic hallucinogens or hallucinogens primarily acting on the 5-HT2a receptors (classic psychedelics). $\endgroup$
    – A. Kvåle
    Feb 27 '21 at 20:44
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    $\begingroup$ Psychedelic and hallucinogen are not necessarily interchangeable. I believe all psychedelics have hallucinogenic properties but not vice versa. Especially if you mean psychedelic in the narrow sense of classic psychedelics. $\endgroup$
    – Bryan Krause
    Feb 27 '21 at 20:45
  • $\begingroup$ @BryanKrause Yes, I too see psychedelics as simply a category within hallucinogens, but I do feel an obligation to point at the different uses of the word. I do state in my "Classification" chapter that in my treatise, that I will almost solely be talking about the classic psychedelics, referring only to said classics when employing the word. Before that though, I just want to clear up what the word means, so that if they read it in different contexts, they understand it may not be used completely identically in that case as in my treatise. [Continued in next comment]. This is no. 1. $\endgroup$
    – A. Kvåle
    Feb 27 '21 at 23:12
  • $\begingroup$ @BryanKrause For example, if I say "psychedelics yada-yada", and it holds true for the classic psychedelics, it may not hold true for a different definition of psychedelics. The readers may read a scientific text using a broader defintion of psychedelics that contains exceptions to whatever I stated in my text, which may prompt the reader to discredit my text. Therefore, I state clearly what I mean when I say "psychedelics", but I also intend to give some info on how the word is used in other contexts, hence my queries about hallucinogen classifications and whatnot. This is no. 2. $\endgroup$
    – A. Kvåle
    Feb 27 '21 at 23:15

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