According to my chemical engineer of a brother, the stuff they put on it is non-toxic but tastes so bad that a drop in a well renders the water undrinkable. Your body won't allow you to drink it
It's called denatonium benzoate and it is the most bitter compound known to man. I once opened a bottle of the solid material and instantly tasted bitterness due to the tiny bit of powder that kicked up into the air from the action of unscrewing the lid. A mere 10 parts per million in water makes it unbearably bitter.
Which I think is awsome.
Also it is only the labal that is coated in it.