In 2010, 88% of chickens raised in the United States received roxarsone, an arsenic-based drug. (Science Daily) The reasons include "promot[ing] growth, treat[ing] disease and improv[ing] meat pigmentation." (Huffington Post - The Arsenic in Your Chicken)
Arsenic is a poison to humans (Wikipedia:Arsenic poisoning).
The above is sufficient to address the quoted claim, but I'll add that this study found that "the use of arsenic-based drugs contributes to dietary [inorganic arsenic] exposure in consumers of conventionally produced chickens", and that "this increase in arsenic exposure could result in 3.7 additional lifetime bladder and lung cancer cases per 100,000 exposed persons", under certain assumptions.