The presence of pain is one of the most common reasons people seek health care. National surveys have found that chronic pain-defined as pain lasting longer than 3 months-affects approximately 100 million American adults and that the economic costs attributable to such pain approach $600 billion annually. Pain has been described in the medical literature as a "uniquely individual and subjective experience" and "among the most controversial and complex" medical conditions to manage. The source of pain for any individual can vary, whether it's an underlying illness such as heart disease or cancer, an injury experienced recently or long ago, or the lingering effects of a medical procedure. Rega