A drug used to treat non-small cell lung cancer that has spread to other parts of the body and has certain mutations (changes) in the EGFR gene. It is used in patients whose cancer has not already been treated with other anticancer therapy. It is also being studied in the treatment of other types of cancer. Gefitinib blocks certain proteins made by the EGFR gene, which may help keep cancer cells from growing. It may also prevent the growth of new blood vessels that tumors need to grow. Gefitinib is a type of tyrosine kinase inhibitor and a type of antiangiogenesis agent. Also called Iressa and ZD1839.