NCI Dictionary of Cancer Terms

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The NCI Dictionary of Cancer Terms features 7,777 terms related to cancer and medicine.

Browse the dictionary by selecting a letter of the alphabet or by entering a cancer-related word or phrase in the search box.

adverse effect
(AD-vers eh-FEKT)
An unexpected medical problem that happens during treatment with a drug or other therapy. Adverse effects do not have to be caused by the drug or therapy, and they may be mild, moderate, or severe. Also called adverse event.