[Suzerain of Sheol]

Okay, so, Wormwood apparently makes water undrinkable because of the taste. People die from it in Revelation because no one can actually drink the water, not because it's poisonous.

That said, I don't really like that idea, so let's say that ingestion is invariably fatal within 24 hours, while contact is not immediately fatal, but will induce sickness to some degree.