Impurity is "any component of a drug substance that is not the chemical entity defined as the drug substance and in addition, for a drug product, any component that is not a formulation ingredient". Impurities in drug substance (i.e., active pharmaceutical ingredient, API) or drug product can arise due to synthetic/manufacturing processes, degradation, storage conditions, container, excipients, or contamination.
They can be identified or unidentified, volatile or nonvolatile, organic or inorganic species. Organic impurities are often process-related or drug-related pharmaceutical impurities. These types of contaminants are most likely to arise during the synthesis, purification, and storage of the drug substance.