Idiosyncratic adverse drug reactions are best described as which of the following?

• A. Common and easily predictable from preclinical studies.
• B. Dose-dependent and predictable from preclinical studies.
• C. Rare, not dose-dependent, and unpredictable from preclinical data.
• D. Always dose-related and predictable.

Answer: (C)

Idiosyncratic adverse drug reactions are rare events that do not depend on the drug dose and cannot be predicted from standard preclinical studies. They arise in a small subset of people, often due to individual factors such as genetic differences or immune-mediated processes, and may only become evident after exposure to the drug in real-world use. Because they’re not dose-related and aren’t detected by typical animal or early human testing, post-marketing surveillance and pharmacovigilance are essential for identifying them. This aligns with the description of idiosyncratic reactions as rare, not dose-dependent, and unpredictable from preclinical data.