During an anaphylactic reaction, the chemicals that are released cause very serious symptoms:
Sudden drop in blood pressure
Narrowing of the airways and swelling of the lips/ tongue which can lead to breathing difficulties
The adrenaline in the EpiPen acts rapidly to treat these potentially life threatening symptoms of anaphylaxis so your blood pressure returns to normal and you can breathe more easily.
It can also help to alleviate other symptoms such as rashes and itch.
It is most effective if given within the first few minutes of a severe allergic reaction.