What constitutes an adverse drug reaction (ADR)?

An adverse drug reaction (ADR) is defined as an undesired side effect or toxicity that occurs as a result of taking a medication. ADRs can manifest in various ways, including unexpected side effects that can range from mild discomfort to severe health complications. They can be the result of normal dosages and often arise from a patient's unique response to a medication, their health status, or interactions with other medications.

The other options do not fit the definition of an ADR: a desired positive effect of a medication is the intended benefit that medication has on the patient, and thus is not considered adverse. An unexpected therapeutic effect, while it may be surprising, is still within the realm of a drug's intended effects and does not qualify as an adverse reaction. Finally, referring to a minor reaction does not capture the full scope of an ADR, which can vary significantly in severity. Therefore, the essence of an ADR is in its nature of being an undesired and unfavorable consequence of medication use.