People with red hair may need more anesthesia, and are often resistant to pain blockers like Novocaine, which could explain why they’re twice as likely to avoid going to the dentist as people with other hair colours, according to new research published in The Journal of the American Dental Association. Their sensitivity to pain could be caused by a mutation in the gene affecting hair colour: in most people, the MC1R gene makes melanin, but a mutation makes a substance called pheomelanin that results in red hair and fair skin, the New York Times reports. (This mutation also occurs in people with brown hair, although it’s less frequent.) In the study, researchers tested for the MC1R gene variant and found it in 65 of 67 redheads, versus 20 of 77 people with brown or black hair. Subjects were then surveyed about fear of the dentist and dental pain. People with the gene variant had more dentist-related anxiety, and were over twice as likely to avoid dental care. “The reason we studied redheads in the beginning, it was essentially an urban legend in the anesthesia community saying redheads were difficult to anesthetize,” said Dr. Daniel I. Sessler, an anesthesiologist who worked on the study. “This was so intriguing we went ahead and studied it. Redheads really do require more anesthesia, and by a clinically important amount.” He suggested that redheads requiring anesthetic talk to their doctor about possible resistance.