Suicide prevention is a major public health concern, with nearly 9,000 suicides and over 200,000 suicide attempts reported each year in France. Suicide attempts and suicidal ideation are among the most frequent reasons for emergency room visits and psychiatric hospitalizations. Although there is no approved pharmacological treatment for suicidal crises, some psychiatric treatments appear promising. Ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist, has shown promising results in reducing suicidal ideation. However, its use is limited due to its side effects. Nitrous oxide, another NMDA receptor inhibitor commonly used in anesthesia and pain management, has demonstrated rapid antidepressant effects and few side effects. Given its rapid and lasting effects, nitrous oxide could swiftly alleviate suicidal ideation.

Inclusion Criteria:

• Active suicidal ideations
• Beck Scale for Suicidal Ideation score greater than or equal to 8
• French speaking
• Patient admitted to psychiatric emergency department
• Capable of wearing a facial mask
• Having signed an informed consent
• Affiliated with social security

Exclusion Criteria:

• Psychotic disorder, neurodegenerative disease, known substance use disorder (excluding caffeine or tobacco), substance intoxication, unstable somatic pathology
• Pregnancy or breastfeeding
• Contraindication to the use of nitrous oxide
• Legal incapacity
• Participation in another drug clinical trial
• Patient subject to compulsory care measures