Don't take vitamin C the day before you go to the dentist. It can make the local anesthetic less effective.
I just read this on a reputable health site and it confirms my own experience. A few months ago I had an old filling replaced, and that morning I took extra vitamin C. The drilling was painful to the point that I had to ask for more novocaine, which I never had to do before. But if you take vitamin C right after the dentist, it will help the novocaine wear off faster.