Suggestion alone can make you feel high. I remember someone telling me that he got high just by touching sweaty people in a music festival where most people were high, clearly, that's placebo. And that guy was far from drug-naive.
And they sometimes give active substances to the control arm to have an effect. I remember in a covid vaccine trial, they gave the meningitis vaccine to the control group to mimic the side effects of the covid vaccine they tried. Here, they actually gave a microdose of psilocybin (1mg) to the SSRI arm. I don't know why, but maybe it was enough for them to feel something without having the full effect.
And they sometimes give active substances to the control arm to have an effect. I remember in a covid vaccine trial, they gave the meningitis vaccine to the control group to mimic the side effects of the covid vaccine they tried. Here, they actually gave a microdose of psilocybin (1mg) to the SSRI arm. I don't know why, but maybe it was enough for them to feel something without having the full effect.