What was the law that required truthful labels?

The Food Additives Amendment was enacted in 1958, 20 years after the original Food, Drug and Cosmetics Act was signed in 1938 by President Roosevelt. Earlier in 1906, the Wiley Act became law, it but was difficult to enforce and eventually became obsolete.

The Food, Drug and Cosmetic Act was amended in 1954 to include the Pesticide Amendment, in 1958 to include the Food Additives Amendment and in 1960 to include the Color Additives Amendment. The original Food, Drug and Cosmetics Act required drug manufacturers to provide proof that new drugs were safe before they were sold, and poisonous substances could no longer be added to food (except where unavoidable or required in production).