Jayne Gumpel, a lead trainer at Fluence and a couples therapist who sees clients in Woodstock, N.Y. and New York City, said the public’s interest in psychedelics “is exploding.” Oregon, Washington, D.C. and a half-dozen municipalities have decriminalized psilocybin, and hundreds of ketamine clinics are popping up in the United States. To stay current, therapists need to have an understanding of these and other psychedelics, including MDMA, Ms. Gumpel added.