Now I’ve legalised cannabis, surely I should have the option to tax it?
Ignore me I’ve just found the option to create the tax for that 
Right click in that area gives the Policy Ideas window for anyone who hadn’t found it like me.
There should be a “legal drugs tax” in policy options. But havent used it much myself. Conservatives, Religions are already angry in most of my games.
However it may be a interesting way to go, since legal drugs consumptions reduces alcohol consumptions, and considering that getting rid of alcohol abuse may be pain, not a bad idea.