Drug resistance refers to a decrease in the effectiveness of a drug. A drug may be ineffective when it is first used, or it may become ineffective over time.
Drug resistance can develop in a few ways. Cancer cells (or microorganisms such as bacteria or viruses) can adapt so that they are no longer killed off by a medication, or those cancer cells or microorganisms that are resistant to the medication in the first place can "take over" after the nonresistant cancer cells or bacteria are killed off.
With lung cancer, research is in progress to develop strategies to eliminate the drug resistance that occurs with some chemotherapy drugs.