For the genus of moth see Iressa (moth)

Gefitinib (ZD1839) (INN, /ɡɛˈfɪtᵻnɪb/, trade name Iressa, marketed by AstraZeneca and Teva), is a drug used for certain breast, lung and other cancers. Gefitinib is an EGFR inhibitor, like erlotinib, which interrupts signaling through the epidermal growth factor receptor (EGFR) in target cells. Therefore, it is only effective in cancers with mutated and overactive EGFR. However, meta-analyses have not shown any improvement in overall survival, that is, it does not extend life.