Gefitinib dihydrochloride is primarily used in the treatment of non-small cell lung cancer (NSCLC), particularly in patients with specific mutations in the epidermal growth factor receptor (EGFR). It works by inhibiting the activity of EGFR, which is often overactive in certain types of cancer, thereby slowing down or stopping the growth of cancer cells. This medication is typically prescribed for patients who have advanced or metastatic NSCLC and whose tumors have tested positive for EGFR mutations. It is administered orally, making it convenient for outpatient treatment. Additionally, gefitinib dihydrochloride is sometimes used in clinical trials to explore its efficacy in other types of cancers where EGFR mutations are present. It is important for patients to undergo regular monitoring to assess the drug's effectiveness and manage any potential side effects.