There is a toxicity linked to the chronic use of nitrous oxide, leading to neurological disorders such as combined sclerosis of the spinal cord. One thus frequently observes patients presenting disorders of walking or paresthesias, of more or less resolving evolution being able to go until the need for using a wheelchair and more recently cases of thrombosis were reported Serum or urine N2O assays are rarely performed routinely, because they do not allow to ensure a real exposure due to the very short half-life of this gas in the body. Thus, other biological monitoring markers are mentioned in the literature, such as vitamin B12 or homocysteine. Unfortunately, there are still no recommendations for biological monitoring of nitrous oxide consumption. Moreover, underlying mechanisms leading to clinical outcomes remains misunderstood.

Inclusion Criteria:

• A current or former recreational nitrous oxide user defined
• With associated clinical signs (group 1)
• Without associated clinical signs (group 2)
• With or without clinico-biological sequelae associated with use
• Consenting to the conduct of the study
• Socially insured

Exclusion Criteria:

• Pregnant or breastfeeding women
• Never used nitrous oxide or stopped using it more than 6 months ago
• Lack of social security coverage
• Not willing to participate in the entire study

Patients using or having recently used nitrous oxide, and hospitalized during the study will be offered to participate in the study.