drugs containing arsenic fed to chickens

Question

Dear Rabbi-- there was a very disquieting story in today's NY Times about arsenic fed to chickens, to kill germs, make flesh pinker--and it wether it can cause cancer in people who eat the chickens. Are these arsenic drugs kosher? Can they be fed to chickens that will be slaughtered according to Halacha for consumption? Thank you for your help-- Mrs. M. Silverman