You could almost OTK somebody with a dragon and six of these at once. I mean, once you've given that jade dragon 12 poison counters this turn, you might as well give him 60 more... :p Plus he still gets to attack once, before experiencing a terrible death to the tune of 72 poison counters. So yeah, love the idea, but really overpowered, I think. You can't change it to creature's hp, either, because that would be -really- overpowered. (hello Titan, shard of focus, voodoo doll... and followed by catapult = LOL) Using creature attack would be roughly as strong as this is now, just with a slightly different set of creatures.
I'm not sure how to balance the card without taking away from your ideas, but I hope you can find a way to do so. The card is so cool, but its coolness is equal to its imbalance. Probably the best thing I can think of to help balance it is, instead of giving the target creature its cost in poison counters, to instead have it kill the creature outright. (like immolation) That way, you can still give someone a huge number of poison counters, but only if you already have a whole bunch of creatures in play to sacrifice, and you can still use it as a way to bypass shields (killing your dragon to poison opponent when they have dimshields or something) or evade CC or whatever. This prevents chaining multiple biotoxins on the same creature, but doesn't ruin the theme and idea of your card.
If you don't believe me that it would be overpowered, I will propose the following change to immolation//cremation: Gain 6//8 fire quanta and 1 of each other quanta. Target creature you own dies at end of turn. You'd be able to sacrifice the same creature to multiple cremations if done all in one turn; my modification prevents a similar abuse of this card.
