Gefitinib impurities are the unwanted substances that can be present in gefitinib drug samples. These impurities can arise during the manufacturing process or from external sources. They can affect the quality and purity of the drug, which can impact its effectiveness and safety. It is essential to monitor and control these impurities to ensure the drug's quality and safety. The impurities can also be used for analytical purposes to identify and quantify the drug and its related substances.