Important pharmacological agents for treatment of chronic neuropathic pain such as ketamine block NMDA receptors. Nitrous oxide, an inhalant agent used extensively In anesthesia, also have antagonist property of NMDA receptors. The investigators hypothesize that prolonged treatment with N2O can alleviate chronic neuropathic pain of different causes (e.g. diabetic neuropathy, post herpetic neuralgia etc.).

Important pharmacological agents for treatment of chronic neuropathic pain such as ketamine block NMDA receptors. Nitrous oxide, an inhalant agent used extensively In anesthesia, also have antagonist property of NMDA receptors. The investigators hypothesize that prolonged treatment with N2O can alleviate chronic neuropathic pain of different causes (e.g. diabetic neuropathy, post herpetic neuralgia etc.). In order to examine our theory we Will recruit 40 patients aged 18-65 that suffer from chronic neuropathic pain. The patients will be divided to two groups of which half will receive treatment with N2O and half with placebo (pure O2) as a repeated weekly treatment for 4 weeks. At the beginning of the each session the patient will be asked to fill questionnaires regarding their pain quality and severity

Inclusion Criteria:

• men and women at the age 18-65
• patient that suffer from chronic neuropathic pain
• VAS>40
• DN4 score >4
• takes pain medication on a regular basis
• Didn't get epidural injection for the pas month
• signed an informed consent form

Exclusion Criteria:

• patients that suffer from chronic lung disease.
• patients with cancer
• patients with heart disease
• pregnant and lactate woman
• patients that suffer from depression