Gefitinib is used to treat non-small cell lung cancer in people who have already been treated with certain other chemotherapy medications and have not improved or whose condition has worsened. Gefitinib has not been shown to help people who have non-small cell lung cancer live longer. There are other medications that may help people who have non-small cell lung cancer live longer. Therefore, only people who have already taken gefitinib and benefited from the medication should continue to take it. People who have never taken gefitinib should start their treatment with a medication that is known to help patients with lung cancer live longer. Gefitinib is in a class of medications called kinase inhibitors. It works by blocking the action of a certain naturally occurring substance that may be needed to help cancer cells multiply.