Is psilocybin an effective antidepressant and what is its Mechanism of action?

Goodwin et al.1 report a single 25 mg dose of psilocybin has an antidepressant effect short-term in medication-resistant depression. Unanswered questions include drug blood level as a guide to dose, psychedelic effects relationship to antidepressant benefit, and potential suicide risk of psilocybin.