I saw this for sale at a poison dealer in Old Town Stormwind.  Not the one in SI7 but the one in the center of the area accessible through an alley.  I bought it just to try it out and used it in WSG at level 29.  I ended up switching back midway through the battle because it is a pointless item.  It does not cause enough damage when it procs to come anywhere near an offhand weapon, especially when coated with poison.  If it had a cool animation like your character spitting out a huge cloud of poison, it might be worth having as a conversation piece or for shock value but it really doesn't do anything.  It's useless.