I have been reading about this a little bit lately. It started with Hartz cat flea products. They put an ingredient in it called phenothrin. This was basically outlawed as a lawn pesticide.
It is very dangerous especially when licked. Quite a few cats died from it. Hartz didn't pull the product after that
but did relabel to make sure owners were applying it correctly. I'm not sure what is wrong with the dog forumula but this ingredient might be in it. I would never ever use Hartz products. They are just not safe.