• A condition that occurs when the body gets used to a medicine so that either more medicine is needed or different medicine is needed.
  NCI Dictionary of Cancer Terms
  U.S. National Cancer Institute, 2021
• A reduction in the sensitivity to a drug following its repeated administration, in which increased doses are required to produce the same magnitude of effect previously produced by a smaller dose. This increase in dose may be necessitated by changes in the metabolism of the drug, or a cellular, physiological or behavioral adaptation to the effects of the drug.
  NCI
  U.S. National Cancer Institute, 2021
• Progressive diminution of the susceptibility of a human or animal to the effects of a drug, resulting from its continued administration. It should be differentiated from DRUG RESISTANCE wherein an organism, disease, or tissue fails to respond to the intended effectiveness of a chemical or drug. It should also be differentiated from MAXIMUM TOLERATED DOSE and NO-OBSERVED-ADVERSE-EFFECT LEVEL.
  NLM Medical Subject Headings
  U.S. National Library of Medicine, 2025
• Tendency of a drug, with repeated use, to become less effective (i.e., require higher dosage) or associated with user dependence; do not use for lack of adverse reactions, as in "the subject tolerated the drug well".
  CRISP Thesaurus
  National Institutes of Health, 2006