Free Cannabis Vocoder VST plugin
It might not have mood-altering properties, but Bitsonic's Cannabis Vocoder does promise to "completely change the sound of your vocal track".
There are two versions of the plugin: mono and poly. The former works as an effect, while the latter is a polyphonic vocoder instrument.
You can find out more on the Bitsonic website, where Cannabis Vocoder can also be downloaded for free for 32-bit Windows systems.