Lists, how to make drop-down lists optional

Is there a way to make a drop-down list optional? When I create drop-down lists the user has to make a choice from that list. Example: a list of doctors. I need the user to be able to select from that list if the patient saw one of those physicians, but if the patient saw another doctor not on the list, I need the user to be able to enter the name of that other doctor. Currently the user has to select from the list or not enter a doctor. I am using Excel 2003.

Re-invoke Data Validation on that cell and in the Error Alert tab turn off show error after invalid data is entered.