This is contained within the NFL Drug policy.
First, these substances threaten the fairness and integrity of the athletic competition on the playing field. Players may use these substances for the purpose of becoming bigger, stronger, and faster than they otherwise would be. As a result, their use threatens to distort the results of games and League standings. Moreover, Players who do not wish to use these substances may feel forced to do so in order to compete effectively with those who do. This is obviously unfair to those Players and provides sufficient reason to prohibit their use.