This value set change was applied during the 2020 VS Update. Built for C-CDA 2.x this value set should be used to document a patient reactant instance by searching through the grouped value sets first in drug ingredient, then drug class, and finally other substance. Once found, any alternative match should not be used as the primary code. Drug class concepts were originally drawn from the NDF-RT code system but current versions use SNOMED CT Drug Product hierarchy if possible and the Substance hierarchy if not found in products. This is to take advantage of the NLM-produced linkage between SNOMED CT drug product concepts and aligned RxNorm drug ingredient (IN,PIN) concepts that can be traced to orderable drugs. The list of drug class concepts used was drawn from the set of drug classes identified in HL7 project to analyze identified allergen and reactant concepts found in existing records published in 2018. This will not be a full list of all drug classes. Note that the content does not include vaccines.