The investigators are conducting a randomized controlled trial to evaluate the antidepressant effects of nitrous oxide in people with Major Depressive Disorder (MDD). MDD is a global medical condition that causes significant health and economic burden. Recent studies have shown that a single dose of ketamine, an NMDA-antagonist, has fast and long lasting anti-depressant effect. Nitrous oxide, another NMDA-antagonist, is widely used for anesthesia and analgesia, safer to administer and has fewer side effects than ketamine.~A randomized controlled crossover feasibility study showed significant reduction in depressive symptoms at 2 and 24 hours after a single 1-hour treatment session of inhaled nitrous oxide compared with placebo. Nitrous oxide is inexpensive and can be safely administered by any trained clinician. If found to be efficacious, it could be used to provide rapid anti-depressant effect whilst the benefit of traditional anti-depressants has its delayed effect. Another potential application could be in acutely suicidal patients.~This trial will enable confirmation and extension of the findings from the feasibility study, and identify the optimal dose and regimen in a broader population of those with MDD. Participants will be randomized to receive a weekly 1-hour inhalational session of either nitrous oxide or placebo (oxygen-air mixture) for 4 weeks, and the nitrous group will be further randomly assigned to a dose of 50% or 25% nitrous oxide. Depression severity and outcomes related to treatment responses will be continuously assessed by a 'blinded-to-randomization' psychiatry (MD) rater at weekly intervals during study patient participation, using validated psychiatric diagnostics (Hamilton Depression Rating Scale-21 [HDRS-21 or HAM-D]; Profile of Mood States [POMS]; Computerized Adaptive Test-Mental Health [CAT-MH]; Sheehan-STS [S-STS]; Visual Analog Scale [VAS]).
To evaluate the acute and sustained antidepressant effects of nitrous oxide in people with major depressive disorder; and further evaluate these effects by identifying the optimal dose and regimen to guide current practice, and to plan a future large pragmatic trial.