What is an unintended, usually predictable effect of a drug called?

A side effect is defined as an unintended, usually predictable effect of a drug that occurs in addition to the desired therapeutic effect. When medications are administered, they interact with various systems in the body; while this can lead to beneficial outcomes, it also often results in secondary effects. These effects can vary in severity and impact, but they are typically known and documented in clinical settings, allowing healthcare providers and patients to be aware of them when considering the use of a specific medication. For example, certain antihistamines may alleviate allergy symptoms but also cause drowsiness as a side effect.

On the other hand, an adverse reaction refers to a harmful or unpleasant response to a medication that is more severe and often unexpected. Complications may arise from the course of an illness or treatment rather than from the medication itself. Tolerance is the body's adaptation to a drug over time, where increasing doses of the medication are required to achieve the same effect. Understanding these distinctions helps healthcare professionals to better inform patients and manage treatment plans effectively.