Postherpetic neuralgia (PHN) is a chronic pain syndrome associated with the reactivation of a primary varicella zoster virus infection and is one of the most serious complication of herpetic zoster. The clinical features of PHN are ongoing pain accompanied by allodynia, hyperalgesia, and paresthesia. Current treatment options aimed at relieving the symptoms of PHN include oral agents, such as, opioid, NSAIDs, antidepressants, anticonvulsants. And local anesthetics with steroids are used for subcutaneous infiltration, somatic nerve block, sympathetic nerve block and epidural nerve block. However, in some cases, the pain does not respond to this treatment. We report a case in which a patient suffering from PHN did not respond to conventional therapy, but in whom continuous intravenous infusion of magnesium and physiologic N-methyl-D-aspartate (NMDA) receptor antagonist, reduced severe pain.