Exceptions to and deviations from the INN common stem system.

For various reasons, mainly related to the history of the drug and of drug nomenclature, some INNs lack a common stem. As INN nomenclature has evolved, some stems have been abandoned or their definition has been modified. Some common stems are difficult to interpret or fail to achieve their objective of designating coherent therapeutic groups. This problem tends to arise particularly when: several stems are used for the same therapeutic use; stems used to designate a particular structure or origin are shared by drugs with different properties; or several stems are used to represent drugs with the same properties but different structures.