Pain can be classified as acute or chronic. The distinction between acute and chronic pain is not based on its duration of sensation, but rather the nature of the pain itself. The primary distinction is: acute pain serves to protect one after an injury whereas chronic pain does not serve this or any other purpose. Drugs in clinical use as analgesics belong to two main groups–narcotic or morphine group and analgesic-antipyretic (non-steroidal anti inflammatory drugs) group. Morphine-like drugs produce analgesia by acting on the central nervous system, while analgesic-antipyretic drugs act by both central and peripheral mechanisms. Animal tests used in the search for new analgesics are designed as models for the treatment of pathological pain in man; but they usually differ from the original in that the drug is given before the noxious stimulus (thermal, electrical, chemical, and mechanical types of stimuli). Hence, these tests only measure the power of a drug to increase the minimal stimulus required to elicit pain or nociceptive response.