I think Carac and Efudex work pretty much the same way. I do know though that I had no reaction to speak of during the first week, only a few red marks on the cheeks it took longer for most of the areas to become affected by the cream. I commend you for getting another opinion and making sure this is right for you. Maybe because your lips are already cracked etc your Derm suspects that your reaction will be faster. I always understood that you had to complete the whole time of 21 days or more of the treatment for the cancer cells to be brought out and killed off????.