The accepted method of classification is based on neurophysiological mechanism, time, etiology, or site of effect. The neurophysiological classification of pain is based on inferred pain mechanisms, and there are two main types: injury-receptive pain and non-injury-receptive pain. Injury-receptive pain is subdivided into somatic pain and visceral pain; non-injury-receptive pain is subdivided into neuropathic pain and psychological pain. According to the duration of pain, it can be divided into acute pain and chronic pain. Acute pain occurs after trauma or surgery and is self-limiting, and is reduced when the tissue damage is recovered. Chronic pain is defined as pain that lasts longer than the normal healing time of an acute injury or disease and recurs at intervals of months or years, or simply as pain that lasts longer than six months. Chronic pain can affect all aspects of life, such as employment, social activities, and interpersonal relationships. The etiological classification focuses more on the primary disease causing the pain, such as cancer pain, arthritic pain, and pain from sickle cell disease. The former accounts for 20%-50% of pain patients. According to the site of onset, it can be divided into headache, shoulder pain, low back pain, and leg pain, etc. In most surveys, low back pain accounts for a large proportion. In the UK, 20% of patients going to hospital are treated for low back pain.