This is NOT a trial that was designed to assess the safety of psilocybin. This was a trial meant to show that a particular dosage of psilocybin is /safe enough/ to pursue further research in that dose range.

This was a /phase one/ clinical trial.

Phase One: administer small doses under very close supervision in a tiny number of people to determine whether it's immediately dangerous, or safe enough /for further study/. They used 89 - initial randomization of 30 each to placebo, 10mg, and 25mg. That's a good number for a phase one trial. But when we look at the sort of things that start hurting people when released to market, they're the sort of adverse effects that go undetected in trials with thousands of participants, and would need an infeasible large trial (>10k patients) to detect.

This trial does not prove safety. This trial is meant to establish a dosing range that is /safe enough/ to participants to allow for further study. They found no "serious" adverse effects, in healthy volunteers - that is, not depressed patients. Can drugs like this have different effects in people w and without depression? Yes they most certainly can, in particular if those people are on concurrent medications (which they will be, since this is being developed for treatment-resistant depression).

“We wanted to look at the safety and tolerability profile of our psilocybin, and to look at the feasibility of a model where up to six one-to-one sessions are held at the same time." No they didn't /want/ to - they /had/ to. This is article is just a press release.

Phase 2 is generally a range of doses, within the range established in phase one, wherein we attempt to determine, to a rough level of accuracy, which doses show toxicity, which show therapeutic benefit, and at roughly what proportions. This phase is meant to determine what dose will be used in a phase 3 trial, if any dose exists with an appropriate proportion of therapeutic benefit to side effect.

Phase 3 - a dose (or couple of doses) of a drug in a regimen meant to reflect real-world proposed usage is distributed and compared against either a currently used drug or a placebo. Depending on how big an effect is expected, how common the disease, etc. we'd look at trials of hundreds to thousands of patients. This is the trial where we get a real sense of safety, side effect profiles, efficacy, etc. This is the make-or-break line for getting your drug to market.

Phase 4 "trial" - aftermarket surveillance. Goes on for years after a drug hits market, because there's no trial that comes to close to capturing the rare side effects that may be visible when applied to the general population.

This was, also, presented to a neuropsychopharm conference, not published data. Which is to say, they're ready to brag about it, but they're not ready to submit themselves to close scrutiny.