Hi, After reading your query I can make out that you are suffering from urticaria. Urticaria is a common condition caused by bursting of allergy producing cells known as mast cells due to certain aggravating factors present in the environment. As the chemical inside them is released it produces a rash which you have mentioned. There can be multiple factors producing this condition which are included in day to day life and some times it is really difficult to find one exact cause. However, Tylenol (acetominophen) does not usually trigger hives or swelling but it can be inducer of welts in limited number of patients, so can be quite possible to get rash because of it. The long term effective treatment for this is by oral anti histamine like loratadine under medical advise. take care.