It's never come up, and probably never will, but in terms of
backstory, yes, it was legalized quite some time ago. There are only a
certain number of drugs not allowed on B5; those which would lead to
destructive, violent behavior that would disrupt the station, and Dust,
about which you won't be hearing for a while.

Near as I can figure, all grass makes you want to do is sit around
eating pizza and watching old Lucy reruns....

jms