Which term describes a reaction that is different from the intended effect of a medication?

The term that accurately describes a reaction that is different from the intended effect of a medication is "side effect." Side effects are unintended responses that occur alongside the desired therapeutic effect of a drug. These can range from mild and common effects, such as drowsiness or nausea, to more severe and rare outcomes.

The intended effects of a medication are considered the primary effects, which are the desired outcomes of treatment. Allergic reactions involve the immune system's response to a medication, resulting in symptoms such as rash or difficulty breathing, and are not merely a deviation from the intended effects but rather an immune response. An overdose occurs when a person takes more medication than prescribed or recommended, leading to toxic effects, which is different from a simple unwanted reaction. Therefore, among the terms provided, side effect is the most accurate descriptor for reactions that are not in line with the desired therapeutic outcome.