I thought I'd summarize and condense all of my thoughts in one post, to make it easier for people to find them. It's not that original, but it's more for anyone who finds the discourse surrounding these drugs very suspicious (a sign your BS-detector is working correctly)-- it's a look at the basic facts and arguments, which you can also use to push back at all the pretentious slop they're trying to inundate social media with.
Getting through to the drug addicts themselves is impossible, because they are members of a massive cult where all they do is jerk themselves off to fantastic tales of wonderful benefits and amazing superpowers and whatnot-- criticizing their preferred flavor of copium threatens to take way their crutch, so they will lash out in fury. But at least you can be aware of how scant the evidence is, and how dubious the reasoning is, and spread that knowledge to others.
1) The proper scientific name for all these substances is 'hallucinogen'-- 'psychedelic' is a literal sci-fi term, invented by the sci-fi writer Aldous Huxley. The former is a more accurate label because it tells you right away what the drugs do (they give you delusions, and make you hallucinate) whereas the latter doesn't and in fact misleads as to their nature.
2) Hallucinogens causes psychosis, especially in people who have a mental-illness or are at risk for one. This is common knowledge. There is also a link to schizophrenia, which is not as simple as stating that they cause it (they may simply bring it out prematurely in those who are genetically predisposed; people with weird brains are also more likely to self-medicate with drugs to cope with their problems), but is there nonetheless. The point is that it's simply not true that they are 'risk free' and 'not harmful'.
3) It is possible to get addicted to them. While hallucinogens themselves don't cause tolerance or withdrawal, people with addictive brains (like drug addicts) can still go through the process of addiction anyways.
It's important to note that in many cases where people repeatedly announce that X or Y 'isn't addictive' they are usually not making a claim about reality or a rational argument or whatever, but are really in the process of beginning the cycle of addiction, and are simply trying to reduce cognitive dissonance by rationalizing their behavior.
You'll notice that this is especially common among users of hallucinogens (not surprising, given that the drugs make you delusional). They will often make verbal claims that are directly at odds with their real-world, observable behavior: they will tell you that the drugs are not addictive, as they withdraw from social relationships in the pursuit of greater and greater highs, or that they're not harmful, as they become more mentally unstable, or that they are not gateway drugs, while getting addicted to all kinds of other drugs. This discrepancy between their self-image and behavior just never occurs to them.
4) Many of the terms confidently used by the hallucinogen enthusiasts don't mean anything, or describe processes that are poorly understood or unclear. In some cases they even imply things that go against basic scientific knowledge.
The confidence with which they use the terms is unwarranted, given how limited the research is, but it is is fairly common among pseudoscientific fads: poorly-understood terms are used to impress laymen and silence their doubts by creating the illusion of total confidence, while the gatekeepers and gurus preen about their special esoteric knowledge (which often turns out to be fabricated).
For example, many of them tout the birth of new neurons ('neurogenesis') that occurs during the drug trip, as if they were sprouting a second brain or enlarging their minds or discovering new things or whatever, when the reality is that this process-- the hippocampus birthing new neurons-- occurs in adulthood as a response to brain damage, such as following traumatic brain injury or a stroke. (This is not to mention that hallucinations themselves are almost always seen as signs of brain damage.)
Of course this is not definitive either, but the point is that the advocates are using poorly-researched terms to justify extraordinary claims, which they also usually present glibly (as if they were well-established facts), when they should require mountains of evidence to prove.
5) Nobody who uses these drugs in the long run comes across as remotely sane, normal, or well-adjusted-- the exact opposite. They suffer from delusions of grandeur and main character syndrome (perhaps due to damaging the parts of their brain responsible for emotional regulation), and to the extent that they can distinguish between reality and their own private narcissistic fantasies, they seem to look down on reality A brief look at their state is a better argument than anything I could come up with.
6) Finally, regular users of hallucinogens are in denial as to who they actually are-- junkies.
This arguably the most damaging aspect of the whole cope, which is that it obscures the nature of what they are doing by presenting their retreat from reality, descent into addiction, and decision to self-medicate with poorly-understood substances as a heroic win, a triumph, a form of exploration, etc when fundamentally none of this is true.
At this point many of them will try to defend their lifestyles by pointing out the many well-known and obvious flaws of Big Pharma and the failures of SSRIs, but in many ways their decisions represent an intensification of the already-dangerous tendencies of the status quo-- they reject the traditional drugs, in favor of even harder versions of them (rather than trying to live a healthy or natural lifestyle) and they replace a flawed process with one that is guaranteed to fail (self-medicating). All in all the whole thing is a very harmful and disturbing trend.