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What is an Adverse Drug Reaction

  • An unwanted or harmful reaction which occurs after it has been used, of a drug or drugs and is suspected or known to be due to drug(s) [MHRA, 2006]. ​

  • A response to a drug that is noxious, unintended, and occurs at doses normally used as a preventative measure, for diagnosis, or treatment of disease, or to improve ones mental health [Tan, 2014].

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Many people think of these as ‘Side Effects’. 


An adverse drug event (ADE) is an injury resulting from medical intervention related to a drug.  This includes medication errors, adverse drug reactions, allergic reactions, and overdoses.


  • In inpatient settings, ADEs:

  • Account for an estimated 1 in 3 of all hospital adverse events

  • Affect about 2 million hospital stays each year

  • Prolong hospital stays by 1.7 to 4.6 days

  • Each year, ADEs in outpatient settings account for:

  • Over 3.5 million physician office visits

  • An estimated 1 million emergency department visits

  • Approximately 125,000 hospital admissions

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