If a commercial feed, customer-formula feed, or contract feed contains a poisonous, deleterious, or nonnutritive substance which is intended for use in the diagnosis, cure, mitigation, or prevention of a disease, or which is intended to affect the structure or any function of the animal body, the Secretary may require the label to show any or all of the following: the amount of the substance present, directions for use, or warnings against misuse of the feed.