Idk if its a bug or intentional, so I wanted to say that right now poisoning basically doesn't work on monsters.
If you get more than the initial tick of poisoning on a monster, before they clear it than you're very lucky
I think making poisoning useful in PvM would be helpful for creating more diversity for what is basically a static build for a PvM character, with poisoning being an option, mages could possibly choose between say anatomy or poisoning, and what they would prefer for use when fighting monsters.
