Epipen is an expendable, pre-filled, mechanical, injecting gadget that contains epinephrine which is used to treat and counter adverse symptoms that arise during allergic reactions. These symptoms may range from hives and itching to rapid heartbeat and low blood pressure, epinephrine counters these symptoms by constricting blood vessels thus reversing low blood pressure and loosening muscles within air passageways to improve breathing patterns and restore regular heart beat.Epipen is an expendable, pre-filled, mechanical, injecting gadget which contains epinephrine and it used to treat anaphylactic or allergic reactions. Symptoms may range from hives and itching to rapid heartbeat and low blood pressure. Epinephrine counters these symptoms by constricting blood vessels. This reverses low blood pressure and loosens muscles within air passageways to improve breathing. EpiPen (epinephrine) is a chemical that works by narrowing the blood vessels and opening the airways in the lungs. This medication causes effects that may reverse severe conditions such as low blood pressure, hives, skin itching, and wheezing caused by severe allergic reactions. EpiPen is mainly used to treat severe allergic reactions, caused by insect stings, drugs, and various other allergens. This medication is also used to treat exercise-induced allergic reactions. However, EpiPen may also be used for other conditions as determined by the doctor. Do not use EpiPen if you are allergic to any ingredient in it. Also do not use this medication if you have asthma, heart disease or high blood pressure.