Stability of drug substances and its testing. The stability of drug substance means resistance to harmful processes occurring both during the production and storage of drugs. The knowledge of factors exerting influence on the stability of drug substance facilitates drug formulation and establishment of storage conditions of the finished products. Stability testing must be conducted according to strictly defined requirements. The tests must be designed in such a manner to reflect changes which the final product will undergo. Abiding by all the requirements during the investigation of stability ensures that the drug will preserve proper quality during the time declared by the producer.