Nitrous Oxide in the Treatment of Depression: A Brief Review

In clinical settings, N₂O is administered as a continually inhaled gas given at varying concentrations in combination with molecular oxygen (O₂). A concentration of 25% N₂O provides sufficient analgesia, and higher concentrations, 30%–50% and up to 70%, are used for sedation and general anesthesia, respectively (6). A dose of 50% N₂O/50% O₂ administered continuously for 1 hour is the standard dosing protocol used in clinical trials investigating the antidepressant effect of N₂O (12). N₂O is rapidly eliminated through the pulmonary system with limited urinary excretion due to poor blood solubility; concentrations measured in exhaled air are 6%–9% and 2%–4% at 5 minutes and 30 minutes postexposure, respectively (6). Recovery to baseline mentation generally occurs within a few minutes postexposure. The pharmacokinetic profile of N₂O makes continuous or repeated use necessary to sustain its effects, lending to the potential for misuse and complicating screening in clinical settings should N₂O-induced sequelae be suspected.


Interest in N₂O stems from its ability to block the N-methyl-d-aspartate receptor (NMDAR) on cortical gamma-aminobutyric acid (GABA) interneurons, a mechanism shared with ketamine (7). Once NMDAR is blocked, regional glutamatergic neurotransmission will increase downstream binding of AMPAR (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor), activation of the mTOR (mechanistic target of rapamycin) pathway, and the release of growth factors such as BDNF (brain-derived neurotrophic factor). Increased synaptic plasticity via long-term potentiation follows, involving increases in the density, efficiency, and overall strength of cortical synapses. These effects are thought to be mechanistically responsible for the rapid antidepressant effects of NMDAR antagonism (13). However, N₂O may preferentially act on opioid receptors at subanesthetic doses, a finding that points toward a potential role of the opioidergic system in modulating its antidepressant effects (12). Importantly, N₂O also interacts directly with the GABAergic, cholinergic, and noradrenergic systems to produce a variety of effects, and further research into its mechanisms is warranted (6).

Four published randomized controlled trials (RCTs) (8–11) have examined the efficacy of N₂O in decreasing symptoms of major depressive disorder or treatment-resistant depression as the primary outcome (Table 1). All trials used inhaled 50% N₂O/50% O₂ for 1 hour as the intervention compared with 1 hour of inhaled 50% air/50% O₂. One of these trials compared 25% N₂O, 50% N₂O, and placebo and revealed a significant decrease in depression symptoms 2 hours postadministration in both N₂O groups but not in the placebo group (9). However, the two N₂O treatment groups did not significantly differ in depressive symptom attenuation. In that study, the 25% N₂O group had a lower rate of adverse effect events. N₂O appears to have the largest effects on remission and response rates among those with major depressive disorder compared with those with treatment-resistant depression (Table 1).

A meta-analysis of these four trials (N=133) (12) revealed that N₂O significantly outperformed placebo in improving depressive symptoms, as measured by the Hamilton Depression Rating Scale (HAM-D), at 2 hours (p=0.001), 24 hours (p=0.001), and 2 weeks (p=0.011) postadministration, with moderate Cohen’s d effect sizes of 0.63, 0.63, and 0.76, respectively. However, results from a separate analysis indicated a significant reduction in depressive symptoms on the HAM-D at 24 hours (p<0.001), with a large standardized mean difference of −2.36 (14).


Results from a recent RCT (N=38; 15) revealed that a 1-hour dose of inhaled 50% N₂O can increase functional connectivity between depression-relevant brain regions among patients with treatment-resistant depression and linked these changes to reductions in depressive symptoms. An increase in global brain connectivity after N₂O administration was also recently demonstrated among healthy participants (16). Another recent RCT (N=34; 17) tested the effect of a single 1-hour dose of 50% N₂O on various neurocognitive domains and reported significant (p=0.002) improvements in executive function at 1 week and secondarily reported significant reductions in HAM-D scores at 2 hours (p=0.005) and 24 hours (p=0.037).


A pooled analysis of three RCTs (N=30) testing N₂O as a depression treatment (18) revealed no effect of N₂O on suicidal ideation at 2 hours postadministration; however, a significant (p=0.019) reduction in suicidal ideation at 24 hours postadministration was identified. In that analysis, a 2-point reduction on item 3 of the HAM-D, which assesses suicidality, was deemed a “meaningful reduction.” At 24 hours, 7 of 13 (54%) participants in the treatment group had a meaningful reduction in suicidal ideation compared with 2 of 17 (12%) in the placebo group. Despite the small sample sizes of the studies, these data are promising, and the role of N₂O in attenuating suicidal ideation warrants continued investigation.


Beyond major depressive disorder and treatment-resistant depression, a recent double-blind RCT of 25 adults with treatment-resistant bipolar depression (19) revealed significant (p<0.001) same-day reductions in symptoms of depression with 20 minutes of inhaled 25% N₂O/75% O₂ and intravenous saline administration compared with inhaled medical air and 2 mg of intravenous midazolam.

Clinical use of N₂O is relatively safe and well tolerated (6). Common adverse effects from clinical or short-term N₂O use include headache, dizziness, nausea, and vomiting. A pooled analysis of four clinical trials (12) reported no significant difference in the incidence of dizziness and headache between the treatment and placebo groups; however, a significantly (p=0.009) higher event rate of nausea and vomiting in the treatment group was found.


Additional problems due to chronic N₂O exposure are closely associated with recreational use of repeated inhaled doses of 100% N₂O and are not often seen in clinical populations (6). Research protocols typically involve few and infrequent administrations of N₂O either once or repeatedly over a few weeks. Given this observation, safety data from chronic exposure to N₂O are also limited to its use in recreational contexts. We note that providers of N₂O treatments are also at risk for chronic exposure to N₂O and the sequelae of such exposure.


N₂O irreversibly lowers vitamin B12 levels, and chronic use may contribute to the development of various neuropathic conditions, including subacute combined degeneration, and may lead to a precarious accumulation of homocysteine, which increases the risk for thromboembolic, cardiovascular, and cerebrovascular events (6, 20). Additionally, recreational use of N₂O may produce transient brain hypoxia and may increase the risk for anoxic brain injuries.


Individuals with the following conditions should not be administered N₂O: pneumothorax, chronic vitamin B12 deficiency, elevated intracranial pressure, bowel obstruction, and middle ear occlusion. Furthermore, individuals with the following conditions were excluded from research protocols to study N₂O administration for depression: recent substance misuse; active serious medical conditions; history of schizophrenia, mania, personality disorder, or obsessive-compulsive disorder; active psychosis; significant or active suicidal ideation; pregnant or breastfeeding; previous treatment with NMDAR antagonists; current treatment with electroconvulsive therapy; and nasal obstruction (8–11).


The clinical investigation of N₂O as a novel therapeutic for major depressive disorder and treatment-resistant depression is supported by replicated demonstrations of its ability to rapidly and robustly attenuate depressive symptoms (8–11, 15, 17). However, available evidence is limited in strength given small participant populations. Concerns about an increase in N₂O use–associated morbidity and mortality rates are valid; however, serious risks are most prominently seen in populations of recreational users rather than clinical participants (6, 20). Recreational misuse of N₂O is worrisome, although concerns about such use should not preclude rigorous scientific investigation of this promising intervention. Taken together, the available evidence supports the clinical use of N₂O as a rapid-acting treatment for depression, which holds promise for forthcoming investigational applications for various psychiatric conditions (21). Further examination of the utility of N₂O in major depressive disorder, treatment-resistant depression, and other mental health conditions should continue to be pursued in clinical trial settings.