Diagnosing a rash caused by a reaction to medicine is complicated. First, a complete review of all prescription and over-the-counter medicines should be done. Even a small amount of a medicine can cause a major reaction in the skin. In addition, the reaction can occur even after you have taken a medicine for a long time. Your healthcare provider will usually advise you to stop taking any medicine that is not needed to sustain your life, to see if the reaction eases. Your provider may give you a substitute medicine, if possible. In some cases, a skin biopsy may be done to help with the diagnosis.