Mayo Clinic -- ROCHESTER, Minn. — Mayo Clinic research shows a correlation between inadequate vitamin D levels and the amount of narcotic medication taken by patients who have chronic pain. This correlation is an important finding as researchers discover new ways to treat chronic pain. According to the Centers for Disease Control and Prevention, chronic pain is the leading cause of disability in the United States. These patients often end up taking narcotic-type pain medication such as morphine, fentanyl or oxycodone.