What is an adverse drug reaction (ADR)?

An adverse drug reaction (ADR) refers to any unwanted or harmful reaction that a patient experiences as a result of taking a medication. This encompasses a wide range of negative effects, which can range from mild discomfort to serious and potentially life-threatening conditions. ADRs can occur even when medications are used as directed and are an important consideration in pharmacology because they directly affect patient safety and treatment outcomes.

Understanding this definition helps clarify why the correct answer is centered on harmful reactions rather than therapeutic effects or minor side effects. Expected therapeutic effects, like those mentioned in the first option, are the desired outcomes of medication use and do not fall under the umbrella of adverse reactions.

While a minor side effect may indeed be troublesome for the patient, it is not classified as an ADR if it doesn't significantly affect the patient's health or if it resolves quickly without intervention. The last option addresses reactions related specifically to dosing errors, which are a specific scenario but not the full scope of what an ADR encompasses.

Thus, the correct response captures the essence of ADRs, emphasizing their undesired and harmful nature, reflecting why safety monitoring is vital in medication management.