Cal. Health & Safety Code § 111280

Any drug is adulterated if it purports to be, or is represented as, a drug that is recognized in an official compendium, and its strength differs from, or its quality or purity falls below, the standards set forth in the compendium. Such determination as to strength, quality, or purity shall be made in accordance with the tests or methods of assay set forth in the compendium, or in the absence of or inadequacy of the tests or methods of assay, those prescribed under authority of this part. No drug defined in an official compendium shall be deemed to be adulterated under this section because it differs from the standard of strength, quality, or purity set forth in the compendium, if its difference in strength, quality, or purity from the standard is plainly stated on the label.