Gefitinib 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) gene. 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 targeted therapy is especially beneficial for patients whose tumors have EGFR exon 19 deletions or exon 21 (L858R) substitution mutations. Gefitinib is typically administered orally and is often considered when other treatments, such as chemotherapy, are not suitable or have been ineffective. It has shown to improve progression-free survival and quality of life in patients with advanced NSCLC. Regular monitoring and management of potential side effects, such as skin rashes and diarrhea, are essential during treatment.