With proper anesthesia root canals can be a pain free procedure. Most teeth respond to regular local anesthesia injections. For Maxillary (upper teeth) dentists usually employ a combination of buccal infiltration and a palatal injection. For Mandibular teeth(lower teeth)a mandibular or mental block is used.
Some teeth do seem to be hypersensitive at the time that root canal is started and patients still can report some sensitivity when a dentist attempts to access the pulp chamber. When this happens I sometimes use an additional intrapulpal injection in order to achieve complete anesthesia. Usually this injection does the trick for my patient and their endodontic procedure can be completed without further discomfort.