What are unintended harmful effects from medications called?

Unintended harmful effects from medications are termed adverse reactions. This term is used in pharmacology and medicine to describe negative and undesired effects that occur as a result of medication use, which may include toxicity, unanticipated effects, or worsening of existing conditions. Adverse reactions can range from mild to severe and may require medical attention or changes in medication.

Other concepts such as side effects, allergic reactions, and drug interactions capture specific types of adverse effects but do not encompass the broader category defined by adverse reactions. Side effects typically refer to predictable and often mild effects that are not the primary purpose of the medication. Allergic reactions are immune responses to a medication that can result in serious, sometimes life-threatening symptoms. Drug interactions occur when the effects of one drug are modified by the presence of another drug, food, or other substances which also leads to unintended effects, but this is specifically related to the interaction between drugs rather than the effects of a single medication.