[b]FDA published a final guidance for pharmaceutical manufacturers that plan to incorporate physical–chemical identifiers (PCIDs) in solid oral dosage forms as an anticounterfeiting strategy. The guidance defines a PCID as “a substance or combination of substances possessing a unique physical or chemical property that unequivocally identifies and authenticates a drug product or dosage form.” PCIDs include inks, pigments, flavors, and molecular taggants.
In the guidance, the agency recommends that the PCID’s ingredients be pharmacologically inactive so that they can be treated as excipients. To minimize toxicological risk, FDA recommends that drugmakers use permissible direct food additives, food substances that are generally recognized as safe (GRAS), or ingredients listed in the agency’s Inactive Ingredient Guide that have been used in solid oral dosage forms.[/b]
http://www.fda.gov/downloads/Drugs/GuidanceComplianceRegulatoryInformation/Guidances/UCM171575.pdf