In 1994, responding to the need for a more useful system for describing chronic pain, the International Association for the Study of Pain (IASP) classified pain according to five axes: region of the body involved (e.g., abdomen, lower limbs), system whose dysfunction may be causing the pain (e.g., nervous, gastrointestinal), temporal characteristics and pattern of occurrence, intensity and time since onset, and etiology. This system has been criticized by Woolf and others as inadequate for guiding research and treatment, and an additional category based on neurochemical mechanism has been proposed.

Duration

Pain is usually transitory, lasting only until the noxious stimulus is removed or the underlying damage or pathology has healed, but some painful conditions, such as rheumatoid arthritis, peripheral neuropathy, cancer and idiopathic pain, may persist for years. Pain that lasts a long time is called chronic, and pain that resolves quickly is called acute. Traditionally, the distinction between acute and chronic pain has relied upon an arbitrary interval of time from onset; the two most commonly used markers being 3 months and 6 months since the onset of pain, 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 durations is "pain that extends beyond the expected period of healing." Chronic pain may be classified as "malignant" (caused by cancer) or "benign" (non-malignant).