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  1. Daniel Vervloet,
  2. Stephen Durham

    Definition

    An adverse reaction to a drug has been defined as any noxious or unintended reaction to a drug that is administered in standard doses by the proper route for the purpose of prophylaxis, diagnosis, or treatment. Some drug reactions may occur in everyone, whereas others occur only in susceptible patients. A drug allergy is an immunologically mediated reaction that exhibits specificity and recurrence on re-exposure to the offending drug.

    Classification of adverse reactions to drugs

    Reactions that may occur in anyone

    • Drug overdose—Toxic reactions linked to excess dose or impaired excretion, or to both

    • Drug side effect—Undesirable pharmacological effect at recommended doses

    • Drug interaction—Action of a drug on the effectiveness or toxicity of another drug

    Reactions that occur only in susceptible subjects

    • Drug intolerance—A low threshold to the normal pharmacological action of a drug

    • Drug idiosyncrasy—A genetically determined, qualitatively abnormal reaction to a drug related to a metabolic or enzyme deficiency.

    • Drug allergy—An immunologically mediated reaction, characterised by specificity, transferability by antibodies or lymphocytes, and recurrence on re-exposure

    • Pseudoallergic reaction—A reaction with the same clinical manifestations as an allergic reaction (eg, as a result of histamine release) but lacking immunological specificity

    Incidence

    Adverse reactions to drugs are very common in everyday medical practice. A French study of 2067 adults aged 20-67 years attending a health centre for a check up reported that 14.7% gave reliable histories of systemic adverse reactions to one or more drugs. In a Swiss study of 5568 hospital inpatients, 17% had adverse reactions to drugs. Fatal drug reactions occur in 0.1% medical inpatients and 0.01% of surgical inpatients. The main drugs implicated are antibiotics and non-steroidal anti-inflammatory drugs. Adverse reactions to drugs occurring during anaesthesia (muscle relaxants, general anaesthetics, and opiates), although less common (1 in 6000 patients receiving anaesthesia), are life threatening, with a mortality of about 6%.

    Numerous mechanisms have been implicated in adverse reactions to drugs. …

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