The effect is a lot slower (and weaker) than plague, so why does it need that additional negative effect? Even with 6, you can't lock the opponent down anyway because every one of his creatures can attack at least twice before dying (once he plays it it attacks, then you poison it, than it attacks and gets damage from poison/dies) With a rush, the few attacks before you get enough of those towers out + two attacks with creatures after that are enough (asuming that all your creatures would die after one round of poison, which wouldn't happen to quite a lot of creatures).
And even if it can target the opponent himself, compared to arsenic (2-4 damage+poison, for 1-2 quanta, weapon) or neurotoxin this card would be fine without that drawback of poisoning you and your creatures when dying.
And even in Poison Dials, this card wouldn't be as good, because you need the opponents creatures to survive and gain nearly nothing from poisoning/killing his creatures.
I think the card is UP as it is now.