During the extraction of a mandibular molar, I split the two roots and extracted the distal one without any pain problem. The extraction of the mesial root was very painful to the patient. The mesial root also had an apical infection with cystic formation. In discussing this case with colleagues, the consensus is that this is a common problem and local anesthesia is not effective on infected regions. Is this true? Do you have any advice for achieving more effective anesthesia in infected areas?