Epinephrine, also known as adrenaline, is a hormone released by the body in a “fight or flight” situation. It is also used as medication for asthma and certain heart conditions, as well as in EpiPens and other auto-injectors used to treat severe allergic reactions.
Because adrenaline can bind to many different cells, its effects are wide ranging. Whether released naturally or delivered through an injection, epinephrine causes people to feel alert and energetic. It also increases the level of sugar in the blood and elevates both heart rate and blood pressure. Epinephrine constricts the blood vessels, redirecting blood to essential organs like the brain and heart. This effect is also what makes epinephrine useful against the swelling that commonly occurs as a result of an allergic reaction. Epinephrine also dilates the air passages, making it easier to breathe.
Epinephrine is released by the body in times of high stress. It serves to prepare the body to take action. Meanwhile, those with severe allergies carry epinephrine auto-injectors because this hormone serves to constrict the blood vessels. This effect slows the immune system's response to the allergen.
In addition to these effects of epinephrine – often considered to be desirable – there are other reactions considered to be less desirable. Epinephrine can cause anxiety, headaches, hypertension, tremors, and heart palpitations. Many people are familiar with these effects when in high-stress situations such as if confronted by an angry person or the prospect of public speaking; those with allergies may also experience them after using an auto-injector following an allergic reaction.