Tell him before, so if he may need anything from where you have been getting the shot you can make sure he has the correct info. I did the same thing and my doc didn't need any information but he did want to call the person who was giving me the shot to make sure I was on time and to make sure he was giving me the shot at the right time. Sometimes doctors want to give you a pap before they give you birth control. Depends on your doctor really. I would give him a heads up though. Plus you dont want to take any chances of not being able to get it when you are suppose to, if he doesn't have what he needs.