Benzidine is a toxic, carcinogenic aromatic amine that can contaminate foods, notably as a manufacturing impurity in some synthetic colorants. It and its derivatives also serve as reagents in food analysis, exploiting their colour reactions. Because of its well-documented bladder-cancer risk in exposed workers, its presence in food-contact materials, dyes, and colours is tightly restricted wherever it is regulated, and reputable colorant manufacturers certify their products free of such impurities. Benzidine exemplifies the hidden-impurity problem in food additives: the named colour may be approved, but manufacturing by-products demand equal scrutiny.