Follow ALL instructions! A moral tail.
This was about 3-4 years ago, and I had a tooth that was really really bad. I went to the dentist and she decided I needed a root canal. She said that tooth looked infected and gave me a prescription for anti-biotics to take for a week before my appointment. For some reason I never bothered to get it filled thinking it would be better to use AFTER the procedure. So the day of, she gave me a couple shots and started drilling. I had heard nightmare stories about root canals so I figured it was supposed to hurt like a mo-fo. She saw me squirming and asked if I could feel it. I said of course, and she said I was not supposed to be feeling anything. She gave me a couple more shots and tried again but I still had total feeling in my tooth. It turns out the bacteria in the tooth was blocking the Novocaine from taking effect, so she eventually had to give me 15 shots to finally numb the area after already drilling on it. Ouch!!!