What type of adverse drug reaction is idiosyncratic when a drug is given in the usual therapeutic dose?

Idiosyncratic reactions are those that occur unpredictably and are not dose-dependent, which means they can happen at therapeutic doses without any apparent relationship to the drug's pharmacological effects. Type B reactions, or idiosyncratic reactions, are characterized by their unusual and often severe nature and can involve immune-mediated responses, metabolic anomalies, or other unique patient-specific factors. These reactions are not related to the drug’s known pharmacological properties and can vary widely between individuals, often making them difficult to predict.

In contrast, Type A reactions are predictable and are usually an extension of the drug's therapeutic effect, commonly related to pharmacological properties. Type C reactions involve chronic effects from long-term use of a drug, and Type D reactions pertain to delayed effects that occur after the drug has been discontinued, neither of which aligns with the idiosyncratic nature of Type B reactions. Therefore, the correct classification of an adverse drug reaction that occurs unexpectedly at a standard therapeutic dose is Type B.