What type of adverse effect is considered an "allergic reaction"?

An allergic reaction is characterized as an immune response triggered by exposure to a medication. This type of adverse effect occurs when the body's immune system mistakenly identifies a substance (in this case, a medication) as harmful and mounts a defense against it. This response can manifest in various ways, such as rash, itching, swelling, or even more severe reactions like anaphylaxis.

In contrast, the other options describe different types of adverse effects. A metabolic disturbance caused by overdose suggests a toxic effect resulting from taking too much of a medication, which is not related to the immune system's response. A side effect that is predictable and dose-related indicates an expected outcome of the medication based on its pharmacological properties, also independent of the immune response. Lastly, a physical dependency on the medication refers to a reliance on the drug due to its effects on the body and does not involve an immune reaction. Thus, the correct choice highlights the specific nature of allergic reactions as immunological responses.