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ClinicalTrials.gov ID

NCT07262008

Lead Sponsor

Women's College HospitalOTHER

Overall Status

Not yet recruiting

Study Type

Interventional

Phase

Phase 3

Enrollment

120Estimated

Last Update Posted

Dec 10, 2025Actual

Start Date

Feb 1, 2026Estimated

Primary Completion Date

Feb 1, 2027Estimated

Completion Date

Sep 1, 2028Estimated

Official Title

Sustained Mood Improvement With Laughing Gas Exposure: A Randomized Controlled Double-Blind Trial

Brief Summary

Multi-centre, parallel group, blinded, 1:1 randomized controlled trial to determine the effect of nitrous oxide on reducing symptoms of depression in patients with treatment resistant depression.

Detailed Description

SMILE is a multi-centre, parallel-group, blinded randomized controlled trial of 120 patients with treatment resistant depression. Consented eligible patients will be randomized to 1) Inhalation of Nitrous Oxide + Intravenous Saline (intervention Group)or 2) Inhalation of Oxygen + Intravenous Midazolam (Active Control Group). Participants will receive their respective study intervention once a week for four weeks for a total of four sessions. Follow-up visits will be conducted over the phone 2-weeks, 4-weeks, and 12-weeks following the last intervention visit. Questionnaires will be administered to assess change in depressive symptoms, function, and quality of life.

• Major Depressive Disorder
• Treatment Resistant Depression

• Inhalation of Nitrous oxide (Laughing gas) + intravenous saline
• inhalation of %100 oxygen + intravenous midazolam

Ages Eligible for Study

Adult (18-64)Older Adult (65+)

Sexes Eligible for Study

All

Accepts Healthy Volunteers

No

Eligibility Criteria

Inclusion Criteria:

1. 18 to 65 years of age
2. Meeting Diagnostic and Statistical Manual for Mental Disorders (DSM-5) criteria for Major Depressive Disorder (MDD)
3. Current major depressive episode as confirmed by the Mini International Neuropsychiatric Interview (MINI) for DSM-5
4. Experiencing moderate to severe depressive episodes, as defined by the Montgomery-Åsberg Depression Rating Scale (MADRS) > 21
5. Failure of two trials of antidepressant therapy of adequate dose and duration, during the current depressive episode
6. For women of childbearing potential, use of highly effective or double-barrier methods of contraception. Abstinence is acceptable if it is the preferred and usual lifestyle of the female participant
7. Capacity to provide informed consent.

Exclusion Criteria:

1. Acute suicidality defined as score > 4 on MADRS item 10
2. Diagnosis of Bipolar Disorder
3. Current substance abuse or dependence and/or history of alcohol abuse or dependence within the past year
4. Dementia
5. Current or lifetime history of schizophrenia or schizoaffective disorder
6. Known history of hypersensitivity or allergy to nitrous oxide, midazolam or any ingredients in the study formulations
7. Contraindication to receiving nitrous oxide (e.g. any condition where air is entrapped within a body and its expansion might be dangerous such as, pneumothorax, elevated intracranial pressure, air embolism, recent middle ear, vitreoretinal or bowel obstruction surgeries, etc.)
8. Known chronic cobalamin or folate deficiency (e.g. signs of anemia or neurological symptoms, with plasma levels of homocysteine over 15 µmol/L and abnormal red blood cells and leukocytes on a complete blood count CBC) or current methotrexate use
9. Contraindication to receiving the placebo midazolam (e.g. shock, chronic heart failure, chronic obstructive pulmonary disease, closed-angle glaucoma, renal failure, patients with limited pulmonary reserve or those with severe decline of vital signs)
10. Daily use of centrally acting medicinal products, such as opioid agonists, (e.g. naloxone and naltrexone) morphine derivatives (e.g. oxycodone, hydrocodone, oxymorphone, codeine), benzodiazepines (e.g. diazepam, clonazepam, alprazolam) and/or other central nervous system depressants such as barbiturates (e.g. phenobarbital, pentobarbital, amobarbital) and alcohol within the past week.
11. Pregnancy or breastfeeding
12. Received electroconvulsive therapy within the past six months
13. Received ketamine treatment within the past six months
14. Received repetitive transcranial magnetic stimulation within the past six months
15. Unwilling to maintain current antidepressant regimen.

Primary Purpose

Treatment

Allocation

Randomized

Interventional Model

Parallel Assignment

Masking

Double

Lead Sponsor

Women's College HospitalOTHER

Collaborators

Anesthesia Clinical Trials Unit (Department of Anesthesia and Pain Management)UNKNOWN

University Health Network, TorontoOTHER

Sunnybrook Health Sciences CentreOTHER

Unity Health TorontoOTHER

Canadian Institutes of Health Research (CIHR)OTHER_GOV

didem.bozak@wchospital.ca4163236008