Yes, it's wormwood.
People used to pour absinthe over a sugar cube. As the cube melted, the sugar turned the drink bright, iridescent green. Supposedly, the combined effect of the alcohol and wormwood was almost like opium, and people got really, really, really f-ed up on it. Yes, it did get addictive-like opium dreams, it becomes a world you really don't want to leave-and it could be pretty deadly because of how raging the addiction became. (Plus, wormwood is poison, so too much of it isn't a good thing.)
I've always wanted to try it, so I'm looking forward even more to getting to the UK!