Full research article:
https://www.thelancet.com/action/showPdf?pii=S2589-5370%2824...
Study design and methods:
> "All the patients provided written informed consent and after discontinuing any pre-trial antidepressants, enrollees received two oral doses of psilocybin (1 mg or 25 mg) with accompaniment from two experienced therapists for ∼6–8 h, separated by 3 weeks, as well as daily pills (escitalopram 10–20 mg or placebo capsules). Thirty patients were randomised to PT and 29 to ET."
> 'The PT condition consisted of two high-dose (25 mg) treatment sessions with the serotonergic psychedelic psilocybin, administered with support from two
study therapists...and daily placebo capsules. The ET condition consisted of daily doses of the selective serotonin reuptake inhibitor (SSRI) escitalopram - 10 mg for three weeks followed by 20 mg for a further three weeks—as well as equivalent psychological support including dosing sessions with placebo-like
doses of psilocybin (1 mg)."
This is an interesting way to address the placebo issue, giving a noticeable microdose of psilocybin (1 mg) versus the active dose (25 mg) for the non-control group.
Fundamentally I think psilocybin's main overall psychological effect is to push subconscious issues up into the mind's conscious processing space. Large doses can generate visual hallucinations related to those subconscious issues which can be unpleasant, even terrifying, for many people, so that's why caution is warranted. Extremely large doses cause complete dissociation from external sensory input, which is of course very dangerous for the unprepared individual in an uncontrolled situation - an experience unlikely for any herbivore to want to repeat, hence the evolutionary selection pressure for biosynthesis of such compounds by plants and fungi.