As used in NRS 639.400 to 639.450, inclusive, ‘product that is a precursor to methamphetamine’ means a product which contains ephedrine, pseudoephedrine or phenylpropanolamine or the salts, optical isomers or salts of optical isomers of such chemicals and may be marketed or distributed lawfully in the United States under the Federal Food, Drug and Cosmetic Act, 21 U.S.C. § 301 et seq., as a nonprescription drug.