I did a google search on this, but got on-topic-results only on fairly old threads and I don't know if what I read was patched/tweaked since then.
The wiki states Poison does damage mobs that are getting healed; some of the old threads state this is false and that it only prevents healing or that it only does weaken the healing effect.
I wasn't paying much attention the times I, in good faith, poisoned everything around a mender, but I can't really remember anything ever dying from it.
Which is it now?