- History of a poor response to 3 or more medications for postherpetic neuralgia (PHN), with poor response defined as treatment with medications in the following categories of therapy for at least 1 month at therapeutic dosages without at least moderate improvement, as judged by the study doctor: Antiepileptic drugs, antidepressants, serotonin norepinephrine uptake inhibitors (SNRIs), opioid analgesics, or lidocaine patch