To assess the effect of 3 consecutive days of one-hour administration of Nitrous Oxide/Oxygen 50%/50% (EMONO) versus placebo as Oxygen/Nitrogen 22%/78% (synthetic medical air), in add-on therapy to chronic analgesic treatments, on average pain intensity in patients with chronic peripheral neuropathic pain. A total of 250 randomised patients to be included in all the participating centres, i.e., 125 randomised patients in each of the 2 study groups treatments