For most drugs, the reactive species contain water, metals and peroxides; it is impurities that affect dosage forms during physical changes and chemical reactions in the system. The purity of the drug substance depends on the reduction of the impurity level, not just the release; lower levels should be maintained throughout the shelf life of the drug substance.
Impurities can be classified into the following categories when the drug substance is synthesized chemically.
- organic impurities
- inorganic impurities
- residual solvents
- Foreign substances and other impurities
- toxic impurities
- ordinary impurities
Therefore, it is necessary to assess the impurities by different instruments and techniques, before assigning the purity value to the drug substance.
