An adverse reaction is a harmful or unwanted reaction to a substance, medication, or treatment. Adverse reactions can range from mild, such as a rash or nausea, to severe, such as anaphylaxis or organ failure. Adverse reactions can occur with any substance, including food, drugs, vaccines, and chemicals. They may be caused by an individual's unique genetic makeup, underlying medical conditions, or interactions with other medications. It is important to report adverse reactions to healthcare providers and regulatory authorities to ensure the safety of future patients and the public.