Roxarsone is an ingredient in chicken feed that contains the element arsenic. It is supposed to promote growth, kill parasites and improve the color of the meat. Under certain conditions the molecule breaks apart and the arsenic roams the body. It is poisonous because it binds to sulphur atoms in enzymes, inhibiting enzyme action. Mercola says that 70% of broiler hens in the US are fed Roxarsone, but that Tyson Foods (the largest poultry producer) has stopped using it.

Some symptoms of arsenic poisoning are a metallic taste in the mouth, diarrhea, vomiting, garlic-like breath, stomach pains, skin lesions and cancers esp of the kidneys, also the liver, lungs and skin.

There's a reason why a chicken caught by the side of the highway in Chile tastes better than one from the grocery store in the US.

Here's Mercola on the biological effects and ramification of arsenic:
http://www.mercola.com/2001/mar/3/arsenic.htm