Chronic pain is pain that lasts a long time. In medicine, the distinction between acute and chronic pain is sometimes determined by an arbitrary interval of time since onset; the two most commonly used markers being 3 months and 6 months since onset, 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'. Epidemiological studies have found that 10.1% to 55.2% of people in various countries have chronic pain. Chronic pain is pain that lasts a long time. In medicine, the distinction between acute and chronic pain is sometimes determined by an arbitrary interval of time since onset; the two most commonly used markers being 3 months and 6 months since onset, 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'. Epidemiological studies have found that 10.1% to 55.2% of people in various countries have chronic pain. Chronic pain may originate in the body, or in the brain or spinal cord. It is often difficult to treat. Various nonopioid medicines are recommended initially, depending on whether the pain originates from tissue damage or is neuropathic. Psychological treatments including cognitive behavioral therapy and acceptance and commitment therapy may be effective for improving quality of life in those with chronic pain. Some people with chronic pain may benefit from opioid treatment while others are harmed. In people with non-cancer pain, a trial of opioids is only recommended if there is no history of either mental illness or substance use disorder and should be stopped if not effective. Severe chronic pain is associated with increased 10-year mortality, particularly from heart disease and respiratory disease. People with chronic pain tend to have higher rates of depression, anxiety, and sleep disturbances; these are correlations and it is often not clear which factor causes another. Chronic pain may contribute to decreased physical activity due to fear of exacerbating pain, often resulting in weight gain. Pain intensity, pain control, and resiliency to pain are influenced by different levels and types of social support that a person with chronic pain receives. The International Association for the study of pain defines chronic pain as pain with no biological value, that persists past normal tissue healing. The DSM-5 recognizes one chronic pain disorder, somatic symptom disorders, a reduction from the three previously recognized pain disorders. The criteria include it lasting for greater than six months. The suggested ICD-11 chronic pain classification suggests 7 categories for chronic pain. Chronic pain may be divided into 'nociceptive' (caused by inflamed or damaged tissue activating specialised pain sensors called nociceptors), and 'neuropathic' (caused by damage to or malfunction of the nervous system). Nociceptive pain may be divided into 'superficial' and 'deep', and deep pain into 'deep somatic' and 'visceral'. Superficial pain is initiated by activation of nociceptors in the skin or superficial tissues. Deep somatic pain is initiated by stimulation of nociceptors in ligaments, tendons, bones, blood vessels, fasciae and muscles, and is dull, aching, poorly-localized pain. Visceral pain originates in the viscera (organs). Visceral pain may be well-localized, but often it is extremely difficult to locate, and several visceral regions produce 'referred' pain when damaged or inflamed, where the sensation is located in an area distant from the site of pathology or injury. Neuropathic pain is divided into 'peripheral' (originating in the peripheral nervous system) and 'central' (originating in the brain or spinal cord). Peripheral neuropathic pain is often described as 'burning', 'tingling', 'electrical', 'stabbing', or 'pins and needles'. Under persistent activation nociceptive transmission to the dorsal horn may induce a pain wind-up phenomenon. This induces pathological changes that lower the threshold for pain signals to be transmitted. In addition it may generate nonnociceptive nerve fibers to respond to pain signals. Nonnociceptive nerve fibers may also be able to generate and transmit pain signals. The type of nerve fibers that are believed to propagate the pain signals are the C-fibers, since they have a slow conductivity and give rise to a painful sensation that persists over a long time. In chronic pain this process is difficult to reverse or eradicate once established. In some cases, chronic pain can be caused by genetic factors which interfere with neuronal differentiation, leading to a permanent reduction in the threshold for pain.