What defines an adverse drug reaction?

An adverse drug reaction refers to an unintended or excessive response to a medication that occurs at therapeutic dosages, meaning it happens even when the drug is taken as prescribed. This definition encompasses a wide range of negative effects that can occur when medications are used, whether they are due to the pharmacological properties of the drug, interactions with other substances, or individual patient characteristics.

The other options describe scenarios that do not fully capture the essence of an adverse drug reaction. An unusual allergic response, while potentially a type of adverse drug reaction, is more specific and does not include all possible negative outcomes from drug use. A change in medication without a prescription is typically considered inappropriate medication management but does not define the reaction itself. Lastly, errors in administering medication focus on the process and could lead to an adverse reaction, but they do not define what an adverse drug reaction actually is. Thus, option B accurately reflects the broader and more inclusive definition of adverse drug reactions.