Definition:
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.
Examples:
Kevin's tumor responded well to the cancer medication he was given, but over time he developed drug resistance and his cancer began to grow again.
