The International Association for the Study of Pain (IASP) classified pain according to specific characteristics:

  1. Region of the body involved (e.g., abdomen, lower limbs),
  2. System whose dysfunction may be causing the pain (e.g., nervous, gastrointestinal),
  3. Duration and pattern of occurrence,
  4. Intensity and time since onset, and
  5. Etiology

According to Woolf, there are three classes of pain :

  1. Nociceptive pain
  2. Inflammatory pain which is associated with tissue damage and the infiltration of immune cells
  3. Pathological pain which is a disease state caused by damage to the nervous system (neuropathic pain, see hereunder) or by its abnormal function (dysfunctional pain, like in fibromyalgia, irritable bowel syndrome, tension type headache, etc.).

Pain is the main reason for visiting the emergency department in more than 50% of case and is present in 30% of family practice visits. Several epidemiological studies have reported widely varying prevalence rates for chronic pain, ranging from 12-80% of the population. A study found that 26% had pain in the last two years of life, increasing to 46% in the last month.