Probably an upper – am I right ? If so and it gets abcessed , the infection can get into the blood stream , too close to the brain to ignore .
I honestly wish I’d never had a root canal , and there have been several . I would have been money ahead to get implants by a good orthodontist . I had one of the best in the country who corrected a botched root canal on a top front incisor . By the time I had him fix it , I had bone loss , so he had to restore the bone first ( six months ) , then insert the socket and let that stabilize for another six months , and finally mount the prosthetic tooth . Other dentists since then have remarked about the excellence of his work .
If I had taken better care of my teeth as a youngster , and put the money saved on later root canals and crowns (and follow-up repairs) into physical PM’s , I’d be a rich man today .