side effect

This term describes an unintended consequence of a medical treatment or a specific action. While it is most frequently used in pharmacology to describe secondary reactions to a drug, it carries a neutral to negative connotation. It differs from an allergy, which is an immune response, by referring to a known or unknown pharmacological effect that occurs alongside the desired therapeutic result.

In non-medical contexts, the phrase is used metaphorically to describe any secondary, often unforeseen result of a policy, decision, or event. In these cases, it often implies a causal chain where the primary goal was achieved, but an additional, typically undesirable, outcome was triggered as a byproduct.