How do you have a placebo dose of psilocybin?! The effects are very noticable so lack of them would quickly reveal it was a placebo, I'd think. Unless it was not a blind test, in which case this would not matter.
Give someone a so-called microdose and it is unlikely that a placebo vs actual study would actually find that people could detect it.
"Psychedelic Effects Subjects scored the 5D-ASC scale at the end of each experimental session. The percent possible score for the total scale was significantly higher after psilocybin (19.35% (7.55)) as compared to placebo (3.08% (1.80); p = 0.026, t(9) = 2.65)"
So, the placebo effect could have caused the reduction in migraines.
The patient doesn't need to take an amount that would get them "high" and the drug is still effective at keeping migraines at bay.
Homeopathy