Epinephrine (or adrenaline) is a critical hormone and neurotransmitter, chemically classified as a catecholamine, that mediates the body's "fight-or-flight" response by stimulating the sympathetic nervous system. It is primarily used as an emergency, self-injectable medication to reverse life-threatening allergic reactions (anaphylaxis) by relaxing airways and increasing blood pressure. It works by constricting blood vessels and increasing heart rate/breathing, with common side effects including anxiety, tremors, and dizziness.