Pharmacological classification, based on a so-called drug challenge test (DCT), is one method to evaluate intractable pain following experimental administration of medicines. In this scenario, the appropriate medicine is prescribed for a patient with intractable pain based on the result of the DCT. A 61-year-old man was healthy until 2003, when he sustained a brachial plexus injury caused by a gas explosion. He had experienced fulgurant pain in his left upper limb since 2006, and had undergone stellate ganglion blocks, and treatment with amitriptyline. He was admitted for further treatment in 2010 because these treatments did not alleviate his fulgurant pain. He could not move his shoulder, elbow, wrist and fingers because of severe paralysis and arthrodesis of his left shoulder joint, and complained of fulgurant pain and persistent dullness in his left upper limb. He was diagnosed to have complex regional pain syndrome (CRPS) due to the brachial plexus injury, and electromyographic examinations showed evidence of a partial regrowth after sural nerve transplantation. Gabapentin was prescribed for the patient's CRPS based on the remarkable effect of thiamilal on this fulgurant pain. The frequency of the fulgurant pain decreased following the initiation of gabapentin therapy, and the persistent dullness disappeared. DCT is recommended for patients with intractable pain such as CRPS, in addition to neurological and electromyographic examinations, in order to elucidate the details of the pain and select the appropriate medication.