Relationships among nitrous oxide exposure, neurological injury and biomarkers

Aims Recreational nitrous oxide (N 2 O) misuse is an increasing public health problem associated with functional cobalamin (B 12 ) deficiency and severe neurological complications. Despite growing recognition of its harms, the dose–response relationship and clinical value of biomarkers remain unclear. This study aimed to characterize the demographics, patterns of use, outcomes, and biomarker utility in patients hospitalized with N 2 O toxicity, as well as to examine the relationship between exposure and toxicity. Methods This retrospective cohort study included hospitalized individuals with N 2 O toxicity (2020–2025) across six Sydney hospitals. Weekly and cumulative N 2 O exposure volumes were estimated. Exposure volumes were compared with clinical outcomes and biomarker results to assess correlations and biomarker utility. Results Eighty‐one individuals across 100 presentations were included. Higher cumulative N 2 O exposure was associated with worse neurological impairment, including subacute combined degeneration of the spinal cord (SCD, p = .006) and peripheral neuropathy (PN, p = .036), and correlated with greater neuropathy severity (Spearman's ⍴ = 0.43, p = .001). Median homocysteine was 50 μmol/L (IQR:32–114;normal 5–15) with a median half‐life of 14.59 h. Median methylmalonic acid (MMA) was 0.68 μmol/L (IQR:0.18–3.16;normal<0.27). B 12 and holotranscobalamin showed high specificity (87% and 98%) but low sensitivity (22% and 5%) for detecting cases associated with SCD, while homocysteine and MMA showed high sensitivity (95% and 93%) but lower specificity (13% and 45%). Conclusions Cumulative N 2 O exposure is the strongest predictor of severe neurological outcomes, demonstrating a dose‐dependent relationship with neurotoxicity. Homocysteine and MMA were sensitive markers associated with neurological toxicity, making them useful screening biomarkers; however, their limited specificity reduces their utility as confirmatory tests. These findings reinforce the importance of clinical assessment and exposure history in diagnosing toxicity.