An unexpected adverse effect of a drug the first time it is given to a patient is considered what type of reaction?

An unexpected adverse effect of a drug upon its initial administration to a patient is classified as an idiosyncratic reaction. This type of reaction is unique to the individual and is not related to the drug's pharmacological actions or dosage.

Idiosyncratic reactions result from a patient's unique genetic makeup, immune response, or other individual factors that can lead to unpredictable responses that are not typically seen in the general population. This distinguishes them from other types of reactions, such as hypersensitivity reactions, which are often predictable responses based on prior exposure or an established immune response.

Understanding idiosyncratic reactions is crucial for healthcare professionals, as it emphasizes the importance of monitoring patients closely when they are administered a new medication for the first time.