inactive ingredient. The list of inactive ingredients is the part of an ingredient label that is not regulated by the FDA other than the requirement that it be a complete list of the contents in descending order of concentration; that is, the ingredient with the largest concentration is listed first, then the next largest, and so forth. Thousands and thousands of inactive ingredients are used in cosmetics, and there is controversy about how truly inactive these substances are in regard to safety as well as about their long-term or short-term effects on skin or the human body.