What is an off-target adverse effect?

• A. The result of the drug binding to a target/receptor for which it was not intended.
• B. An adverse effect caused by reduced drug metabolism in elderly patients.
• C. An adverse effect resulting from the drug binding to its intended therapeutic target.
• D. An adverse effect unrelated to receptor binding.

Answer: (A)

Off-target adverse effects come from a drug interacting with receptors or targets other than the one intended for its therapeutic effect, producing unintended biological responses. That’s why the best description is the drug binding to a receptor it was not meant to interact with, leading to side effects.

Context helps: when a drug hits unintended targets, it can cause symptoms not related to its primary purpose. By contrast, adverse effects due to reduced metabolism in elderly patients reflect pharmacokinetic changes rather than off-target receptor interactions. Adverse effects from binding to the intended therapeutic target are on-target effects, not off-target. And an adverse effect unrelated to receptor binding would imply a mechanism not driven by receptor interactions, which isn’t what “off-target” describes.