The term moiety should be reserved to describe the larger characteristic parts of molecules and not used to describe smaller functional groups, which are made up of atoms that participate in similar chemical reactions in most molecules that contain them. In some instances moieties may be composed of yet smaller moieties and functional groups.
Moieties that constitute branches extending from the backbone of a hydrocarbon molecule, which can often be broken off and substituted with others, are called substituents or side chains.
^"Electronic Code of Federal Regulations Title 21: Food and Drugs § 314.3". Electronic Code of Federal Regulations. United States Government Publishing Office. 22 January 2019. Retrieved 15 February 2019. Active moiety is the molecule or ion, excluding those appended portions of the molecule that cause the drug to be an ester, salt (including a salt with hydrogen or coordination bonds), or other noncovalent derivative (such as a complex, chelate, or clathrate) of the molecule, responsible for the physiological or pharmacological action of the drug substance.