Sorry, I can't answer, but I am interested in hearing from someone who can. I had some basal cell cancer removed from my face via surgery and then was told to use efudex on the rest of the face (4 weeks). Since my chest gets substantial sun like my face, and I have often discovered "spots" that concerned me, I decided to do that as well. After a couple of weeks my face was a wreck, but no reaction whatsoever on the chest. I read in the instructions that if you don't have any reaction after two weeks, you just stop. If there is something about reactivity in different areas of the body, I would like to know that.