What is an idiosyncratic drug response?

• A. A common adverse effect observed in most patients
• B. An unusual drug response or one that is observed infrequently in most patients
• C. A predictable, dose-dependent effect
• D. An immune reaction occurring rapidly after exposure

Answer: (B)

An idiosyncratic drug response is an unusual, unpredictable reaction that occurs only in a small subset of patients and is not related to the dose or the drug’s normal pharmacology. That’s why the best description is an unusual response observed infrequently in most patients—the reaction isn’t something you’d anticipate from typical pharmacology or common side effects.

This differs from common adverse effects, which occur in most people and are often dose-related. It also differs from a predictable, dose-dependent effect, which follows the drug’s known pharmacologic action. And while some idiosyncratic reactions can involve the immune system, they aren’t defined by rapid, typical immune hypersensitivity; they can be immune-related or not, and they tend to be rare and idiosyncratic rather than universal or promptly predictable.