A technique whereby a patient can deliver an analgesic substance (see ANALGESICS) in amounts related to the extent of the PAIN that he or she is suffering. For example, to combat post-operative pain, some hospitals use devices which allow patients to give themselves small intravenous amounts of opiates when they are needed. Pain is more effectively controlled if it is not allowed to reach a high level, a situation which tends to happen when patients receive analgesics only on ward drug rounds or when they ask the nursing staff for them.