Intractable chronic pain is very difficult to treat. Nowadays, small amounts of drugs, that have different actions on the mechanism of pain relief are administered intravenously, and the effects of the test drugs on individual chronic pain patients are investigated by using the evaluation method of the visual analogue scale (VAS). This will enable elucidation of the mechanisms of pain in each chronic pain patient. Based on this information, drugs that are effective for the treatment of individual chronic pain patients can be prescribed. Drugs that are used for the drug challenge tests are phentolamine, barbiturate, morphine, lidocaine, ketamine, benzodiazepine, adenosine-3-phosphate (ATP), neurotropine, and prostaglandine E1. Phentolamine is effective for the management of sympathetically maintained pain. Barbiturate and morphine are effective for the treatment of deafferentation pain and nociceptive pain, respectively. Lidocaine is effective for the treatment of neuropathic pain; ketamine, for allodynia; and benzodiazepine, for anxiety-related pain. ATP exerts a positive effect in total pain management. Neurotropine and prostaglandine E1 are effective for the management of neuropathic pain and ischemic pain, respectively. These tests aid in the selection of drugs that maybe useful for the treatment of intractable chronic pain in patients.