Determination of the Antidepressant Duloxetine in Plasma to Improve the Knowledge of the Analgesic Action of Antidepressants on Chronic Neuropathic Pain

The primary purpose of this protocol is to evaluate the lowest plasma concentration (ie before the daily taking dose of duloxetine) in patients relieved of at least 30% of their neuropathic pain with duloxetine treatment. The secondary purpose of this protocol is to determine plasmatic concentration peak (ie 6 hours after taking duloxetine) in patients relieved of at least 30% of their neuropathic pain with duloxetine treatment. Others secondary purposes are to evaluate the intensity of neuropathic pain, to assess the degree of pain relief and to evaluate the sensation of the global improvement experienced by the patient.