leggendo erowid.org la pericolosità sembra dipendere dal tipo di ortensie usate:

https://www.erowid.org/herbs/hydrangea/

Diuretic and Soothant. Hydrangea paniculata has been smoked to produce cannabis-like effects, but many species from this genus are dangerous if smoked, as they contain a cyanide compound when burned.

che peccato