Chronic pain is defined as pain that has lasted longer than three to six months, though some theorists and researchers have placed the transition from acute to chronic pain at 12 months. Others apply acute to pain that lasts less than 30 days, chronic to pain of more than six months duration, and subacute to pain that lasts from one to six months. A popular alternative definition of chronic pain, involving no arbitrarily fixed duration, is “pain that extends beyond the expected period of healing”. There is little evidence for treating most types of chronic pain with opioids. An exception is chronic pain due to cancer. While they may improve pain in the short term there is no evidence of improved long term pain or functioning. Risks include overdose and addiction. In the United States about 100 million people have chronic pain, with 25% of those having more or severe chronic pain.