Iressa is a prescription medicine that is used to treat certain types of lung cancer. Iressa is in a class of anti-cancer medications called epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors. The way Iressa works is not fully understood. It is thought to work by blocking the action of a certain naturally occurring substance that may be needed to help cancer cells grow and multiply. Iressa blocks a protein called epidermal growth factor (EGF) from triggering growth and multiplication of cells, including cancer cells. Normally when EGF attaches to cells on epidermal growth factor receptors (EGFRs), it causes another substance called tyrosine kinase to become activated. Tyrosine kinase triggers chemical processes that cause cells, including cancer cells, to grow and multiply.