Which reaction is characterized by an unexpected effect of a medication that is not allergic?

The idiosyncratic reaction is characterized by an unexpected effect of a medication that is not related to an allergic response. This type of reaction occurs due to individual variation in the way a person metabolizes or responds to a drug, which can result in effects that differ significantly from what is typically anticipated. For instance, a medication might normally cause sedation in the general population, but in an individual with a specific genetic makeup, it could lead to increased alertness.

Paradoxical reactions, while also unexpected, specifically refer to effects that are the opposite of what is intended—for example, a calming medication causing agitation in some individuals. Systematic side effects are generally expected and consistent with the drug's pharmacological profile and can occur in a predictable manner for many individuals. Drug tolerance refers to a decreased response to a medication after repeated use, meaning that the expected effect becomes less pronounced over time.

In summary, an idiosyncratic reaction is uniquely tied to individual differences and is not consistent with the typical effects of the medication, making it the most appropriate choice for the scenario described.