A patient with no documented drug allergies experiences a highly unpredictable response to a drug. ...

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An idiosyncratic reaction is a highly unpredictable response that may be manifested by overresponse, underresponse, or an atypical response. Because of its rare occurrence, it is impossible to predict beforehand the changes of this type of reaction. An adverse effect is a serious reaction by the patient to the medication and is generally sufficient reason for the medication to be discontinued. A drug allergy is an antigen-antibody immune reaction that occurs when an individual who has been previously exposed to a drug has developed antibodies against the drug. Drug tolerance occurs when the body becomes so accustomed to a specific drug that larger doses are needed to produce the desired therapeutic effect.