Gefitinib is a medication used primarily to treat non-small cell lung cancer (NSCLC) that has specific mutations in the EGFR gene. It works by inhibiting the activity of the epidermal growth factor receptor, which is involved in the growth and spread of cancer cells. By blocking this receptor, gefitinib can help slow down or stop the progression of the disease. Patients typically take gefitinib as a daily oral tablet. It is important for individuals to undergo genetic testing to determine if their cancer has the EGFR mutation, as gefitinib is most effective in those cases. Common side effects may include skin rashes, diarrhea, and liver function changes.