What is the evidence for safety and efficacy of methionine for treating patients with nitrous toxicity with elevated homocysteine concentrations?

Comment by InpharmD Researcher

Literature evaluating the safety and efficacy of methionine for nitrous oxide toxicity with elevated homocysteine concentrations is limited and consists primarily of case reports, with no comparative clinical trials establishing benefit. Methionine has been used as an adjunct to vitamin B12, commonly at 1 g orally three times daily for approximately 2–6 weeks, with several reports describing neurologic improvement; however, concomitant vitamin B12 therapy, nitrous oxide cessation, and other supportive interventions prevent determination of methionine-specific efficacy. Although elevated homocysteine is common in nitrous oxide toxicity, evidence that methionine specifically lowers homocysteine is lacking; one small controlled study found that preoperative methionine enhanced methionine synthase recovery but did not reduce the nitrous oxide-associated homocysteine increase compared with control, while one case report documented normalization of homocysteine during combined vitamin B12 therapy and a methionine-rich diet. Overall, evidence primarily supports vitamin B12 and nitrous oxide cessation as the mainstays of treatment; methionine has been used as adjunctive therapy, but evidence is insufficient to establish its independent efficacy or safety, particularly in patients with elevated homocysteine concentrations.
Background

A 2023 review article discussing the diagnosis and management of nitrous oxide toxicity explains that nitrous oxide causes a functional vitamin B12 deficiency by irreversibly oxidizing methylcobalamin and inhibiting methionine synthase. This impairs the conversion of homocysteine to methionine, resulting in elevated homocysteine levels and reduced methionine production, with downstream effects on myelin synthesis and DNA production. For treatment, the authors identify cessation of nitrous oxide exposure and vitamin B12 supplementation as the mainstays of therapy. Experts also state that vitamin B12 may sometimes be given in combination with methionine and recommend methionine 1 g orally 3 times daily for at least 4-6 weeks or until significant clinical improvement. The authors describe methionine as safe but explicitly note that evidence supporting its efficacy is limited. Importantly, the review does not provide clinical evidence demonstrating that methionine supplementation specifically reduces elevated homocysteine concentrations or improves clinical outcomes in patients with nitrous oxide toxicity. Thus, while the pathophysiology provides a rationale for methionine supplementation, the available evidence described in this review supports its use only as an adjunct to vitamin B12 replacement rather than establishing its efficacy for treating hyperhomocysteinemia. [1]

A 2023 review examines the recognition, investigation, and treatment of nitrous oxide-induced subacute combined degeneration (N₂O-SACD). Nitrous oxide inactivates vitamin B12, impairing the conversion of homocysteine to methionine; homocysteine concentrations are elevated in approximately 80%–90% of reported N₂O-SACD cases and may normalize with vitamin B12 treatment and N₂O abstinence. The authors identify methionine as a possible adjunctive treatment but state that there is no clinical evidence supporting its routine use and anecdotally report that they do not use methionine in their usual, clinical practice. The recommended treatment approach consists of N₂O cessation and intramuscular hydroxocobalamin, with folate measurement and replacement when deficient. [2]

A 2026 review evaluates the detection and treatment of adverse events associated with chronic recreational N₂O use. The review notes that the Royal Children’s Hospital Melbourne recommends methionine 1 g orally three times daily for 2 weeks, while a Canadian protocol recommends 1 g orally three times daily for 4–6 weeks or until significant symptomatic improvement; none of the guidelines or protocols reviewed specifically recommend against methionine use. However, evidence supporting methionine is limited to case reports, and methionine added to vitamin B12 has not been directly compared with vitamin B12 alone in patients with chronic N₂O-related adverse events. [3]

A 2022 systematic review assesses the clinical features, investigations, treatment, and outcomes of chronic N₂O toxicity using 24 case series and 91 case reports. Homocysteine is frequently elevated, including 187 of 208 reported values in case series and 49 of 53 reported values in case reports, and is identified as an important biomarker of functional vitamin B12 deficiency. Methionine replacement was reported in 2 case series and 4 case reports; one case series administered methionine 1 g orally three times daily for 1–2 weeks to 11 patients, while another reported methionine use in 15 patients. The review notes that direct methionine replacement has been proposed to bypass impaired methionine synthase activity, but available evidence is very low quality, there are insufficient data to assess the effectiveness of methionine or other therapies, and further research evaluating vitamin B12 and methionine treatment is needed. [4]

Background References: [1] De Halleux C, Juurlink DN. Diagnosis and management of toxicity associated with the recreational use of nitrous oxide. CMAJ. 2023;195(32):E1075-E1081. doi:10.1503/cmaj.230196
[2] Paris A, Lake L, Joseph A, et al. Nitrous oxide-induced subacute combined degeneration of the cord: diagnosis and treatment. Pract Neurol. 2023;23(3):222-228. doi:10.1136/pn-2022-003631
[3] White CM, White LR. Nitrous oxide abuse prevalence, mechanisms, treatments and prevention. Curr Med Res Opin. 2026;42(3):451-459. doi:10.1080/03007995.2026.2670004
[4] Marsden P, Sharma AA, Rotella JA. Review article: Clinical manifestations and outcomes of chronic nitrous oxide misuse: A systematic review. Emerg Med Australas. 2022;34(4):492-503. doi:10.1111/1742-6723.13997
Literature Review

A search of the published medical literature revealed 6 studies investigating the researchable question:

What is the evidence for safety and efficacy of methionine for treating patients with nitrous toxicity with elevated homocysteine concentrations?

Level of evidence

D - Case reports or unreliable data  Read more→


Please see Tables 1-6 for your response.

 

Preoperative Methionine Loading Enhances Restoration of the Cobalamin-dependent Enzymе Methionine Synthase after Nitrous Oxide Anesthesia

Design

Small prospective controlled interventional study

N= 14

Objective

To investigate the possible protection against cobalamin inactivation by preoperative methionine loading in patients undergoing nitrous oxide anesthesia

Study Groups

Oral methionine loading (n= 7)

Control group (n= 7)

Inclusion Criteria

ASA physical status I or II, undergoing otolaryngologic surgery, receiving nitrous oxide anesthesia

Exclusion Criteria

Not reported

Methods

Patients were allocated using a pre-randomized list to receive either methionine 100 mg/kg in 150 mL orange juice 2 hours before anesthesia or 150 mL orange juice without methionine as the control. Anesthesia was maintained with 70% nitrous oxide in 30% oxygen and isoflurane for 75–230 minutes; mean nitrous oxide exposure was 150 minutes in the methionine group and 162 minutes in the control group. Fasting blood samples were collected before methionine administration, immediately before and after nitrous oxide exposure, and on postoperative days 1, 2, 3, 5, and 7. Assessments included methionine synthase and methylmalonyl-CoA mutase activity in mononuclear white blood cells, total plasma homocysteine, plasma methionine, serum methylmalonic acid, cobalamin, folate, creatinine, and hemoglobin. Three healthy subjects who received methionine without anesthesia were evaluated using a similar protocol.

Duration

Follow-up: 7 days

Outcome Measures

Methionine synthase activity in mononuclear white blood cells; total plasma homocysteine concentrations; methylmalonyl-CoA mutase activity; serum methylmalonic acid concentrations; plasma methionine concentrations

Baseline Characteristics

 At baseline, all patients had erythrocyte folate, serum folate, serum cobalamin, and serum creatinine values within reference ranges. Mean total plasma homocysteine was 9.2 µmol/L (range, 5.2–13.1), mean serum methylmalonic acid was 0.07 µmol/L (range, 0.02–0.11), and mean plasma methionine was 25.6 µmol/L (range, 13.7–33.6). There were no significant differences in these parameters between the methionine and control groups. Serum folate (r= −0.73; p< 0.005) and serum cobalamin (r= −0.61; p< 0.05) were negatively correlated with total plasma homocysteine.

Results

Nitrous oxide exposure resulted in significant methionine synthase inactivation in both groups, with nadir activity of 42% (standard deviation [SD] 15%) of preoperative levels in controls and 40% (SD 10%) in methionine-treated patients; the decline did not differ between groups (p> 0.5).

Recovery was faster with methionine: by days 5–7, methionine synthase activity equaled baseline in 3 patients and exceeded baseline in 4 patients, whereas no control patient returned to baseline within 7 days; between-group differences were significant at days 5 and 7 (p< 0.03).

In controls, plasma homocysteine increased significantly after nitrous oxide exposure and remained above baseline at day 7, with a mean increase of 28.7% (SD 8.9%; p= 0.03). Methionine loading produced a marked transient increase in plasma methionine and homocysteine; at day 7, homocysteine remained increased by a mean of 30.5% (SD 21.6%), which did not significantly differ from controls (p≥ 0.5). No significant changes in methylmalonyl-CoA mutase activity or serum methylmalonic acid were observed after nitrous oxide anesthesia or methionine loading combined with nitrous oxide anesthesia.

Adverse Events

 All patients except one were free of persistent nausea or vomiting and had normal postoperative food intake; treatment-related adverse events were not specifically reported.

Study Author Conclusions

This data suggest that short-term exposure to nitrous oxide selectively impairs the function of the cobalamin-dependent methionine synthase. Furthermore, preoperative administration of methionine should be considered as a means to counteract adverse effects of nitrous oxide.

Critique

This was a small study with short-term follow-up and was designed to evaluate biochemical effects rather than clinical efficacy or safety. Patients had normal baseline homocysteine concentrations and received methionine before nitrous oxide exposure, limiting applicability to patients with established nitrous oxide toxicity and elevated homocysteine. Adverse events were not systematically assessed or reported.



Table 1 References:
[5] Christensen B, Guttormsen AB, Schneede J, et al. Preoperative methionine loading enhances restoration of the cobalamin-dependent enzyme methionine synthase after nitrous oxide anesthesia. Anesthesiology. 1994;80(5):1046-1056. doi:10.1097/00000542-199405000-00014

 

Methionine in the treatment of nitrous-oxide-induced neuropathy and myeloneuropathy

Design

  Case series

Case 1

A 36-year-old man presented with progressive paresthesias, impaired coordination, difficulty walking, Lhermitte sign, and erectile dysfunction after inhaling approximately 48–72 nitrous oxide cartridges nearly daily for 6 months. Neurologic examination showed lower-extremity hyperesthesia and hyperalgesia, impaired vibration and position sense, severe sensory dystaxia, absent ankle reflexes, and bilateral Babinski signs. Laboratory testing showed hemoglobin 6.8 g/dL, hematocrit 20.2%, MCV 106.5 fL, and serum vitamin B12 90 pg/mL; serum folate was 9.5 ng/mL.

Intramuscular vitamin B12 was initiated, with significant hematologic improvement, although neurologic deterioration continued initially, including progression of paresthesias, fecal and urinary incontinence, hyperreflexia, ankle clonus, and lower-extremity weakness.

Neurologic improvement began approximately 4 weeks later, and by 4 months after symptom onset and last nitrous oxide use, the patient could walk with some hesitation and had regained bowel, bladder, and sexual function, with residual mild neurologic abnormalities.

Case 2

A 32-year-old woman presented with progressive paresthesias, gait instability, inability to walk, and an episode of urinary incontinence after recreational nitrous oxide use for 3 months, up to 200 cartridges/day 3–4 times per week. Laboratory testing showed hemoglobin 7.0 g/dL, hematocrit 20.5%, MCV 108 fL, serum vitamin B12 92 pg/mL, and serum folate 195 ng/mL.

Intramuscular vitamin B12 was initiated immediately; her hematologic abnormalities gradually improved, but her neurologic condition continued to progress, with development of spasticity, brisk reflexes, and bilateral Babinski signs.

On hospital day 7, a methionine-supplemented diet, derived by modifying and reversing a homocystinuria diet, was initiated, after which her condition began to improve, with progressive improvement in dystaxia, spasticity, and sensory symptoms. She was able to walk without assistance 2 weeks later and had only minimal appendicular dystaxia 6 months after her initial symptoms.

Study Author Conclusions

 In conclusion, our cases showed more marked hematological involvement and B12 depression that those previously reported, offering further evidence that the neurotoxic effects of nitrous oxide are mediated by interaction with vitamin B12. We cannot prove that the patient who received methionine supplementation improved as a result of this. However, we think that there are sufficient theoretical reasons to encourage the use of methionine as an inexpensive and safe initial treatment for nitrous oxide intoxication. 
Table 2 References:
[6] Stacy CB, Di Rocco A, Gould RJ. Methionine in the treatment of nitrous-oxide-induced neuropathy and myeloneuropathy. J Neurol. 1992;239(7):401-403. doi:10.1007/BF00812159

 

Nitrous Oxide Toxicity: Case Files of the Carolinas Medical Center Medical Toxicology Fellowship

Design

  Case report

Case presentation

 A 45-year-old man presented with progressively worsening paresthesias and numbness of the distal extremities after inhaling approximately 100 nitrous oxide canisters per week for 4 weeks. Neurologic examination showed bilateral upper- and lower-extremity sensory deficits and weakness, absent vibration sense in the distal lower extremities, dysmetria, slowed rapid alternating movements, positive Romberg test, and an ataxic gait preventing independent ambulation. Laboratory testing showed vitamin B12 154 pg/mL, folic acid >24.8 ng/mL, homocysteine 178.2 μmol/L (reference range, 6–15 μmol/L), and methylmalonic acid 1145 nmol/L (reference range, 0–378 nmol/L). MRI demonstrated T2 hyperintense signal involving the dorsal columns of the cervical and thoracic spinal cord.

The patient received cyanocobalamin 1000 μg intramuscularly daily for 14 days and empiric intravenous thiamine for 5 days. Because methionine supplementation was unavailable in the hospital, nutrition was consulted to provide a methionine-rich diet. By day 11, homocysteine had decreased from 178.2 to 9.5 μmol/L, within the reference range, and vitamin B12 was >1500 pg/mL; his ataxic gait, impaired position and vibration sense, and dysdiadochokinesia had significantly improved, and he could ambulate independently, although he had not returned to baseline.

He was discharged on day 14 with weekly intramuscular cyanocobalamin 1000 μg for 1 month and was advised to take over-the-counter methionine 1000 mg orally once daily until symptom resolution.

Study Author Conclusions

 Cessation of nitrous oxide use and intramuscular vitamin B12 repletion are the mainstays of treatment for nitrous oxide-induced myelopathy. The authors note interest in methionine supplementation and report that animal studies and limited human case reports describe improved neurologic outcomes with methionine in addition to vitamin B12. In their patient, a methionine-rich diet was provided during hospitalization, and methionine 1000 mg orally once daily was recommended at discharge until symptom resolution.
Table 3 References:
[7] Shah K, Murphy C. Nitrous Oxide Toxicity: Case Files of the Carolinas Medical Center Medical Toxicology Fellowship. J Med Toxicol. 2019;15(4):299-303. doi:10.1007/s13181-019-00726-x

 

Nitrous Oxide-Induced Myelopathy: A Case From French-Speaking Switzerland

Design

  Case report

Case presentation

A 27-year-old man presented with rapidly progressive paresthesias of both lower limbs and hands, worsening unsteadiness, and occasional lower-limb weakness after heavy recreational nitrous oxide use. He reported a 7–10-year history of nitrous oxide use, occurring 2–5 times per month at 5–20 canisters per episode, with heavy use the night before symptom onset. Neurologic examination showed marked sensory ataxia predominantly affecting the lower limbs, absent vibration sense up to the iliac crests, severely impaired joint position sense at the toes and ankles, globally decreased reflexes, a positive Romberg test, and a profoundly ataxic gait. Laboratory testing showed vitamin B12 139 pmol/L, methylmalonic acid 4.94 µmol/L, and total homocysteine 54.4 µmol/L (reference range, 5.0–15.0 µmol/L). Cervical MRI demonstrated dorsal column T2 hyperintensity from C1 to C5 with an inverted V pattern.

Treatment consisted of hydroxocobalamin 1 mg subcutaneously daily for 7 days, followed by oral cobalamin 2 mg daily until symptom resolution. Methionine 1 g three times daily was started for 6 weeks, and folate supplementation was initiated concomitantly with vitamin B12 therapy. Physiotherapy for gait re-education and proprioceptive training was also provided. At 3-month follow-up, the patient reported that he had discontinued vitamin B12 treatment and resumed nitrous oxide use; persistent paresthesias extending to the knees and a markedly ataxic gait were reported.

Study Author Conclusions

 This case represents a rare and likely underreported occurrence of N₂O-induced myelopathy in French-speaking Switzerland, emphasizing the need for greater clinical awareness. It illustrates the need for increased clinical awareness and early recognition of this reversible but potentially disabling condition. Because the symptoms are subtle and may mimic other causes of sensory ataxia, detailed history taking, especially regarding recreational drug use, is critical. Public health interventions and education targeting young adults are equally necessary to curb this preventable form of neurotoxicity.
Table 4 References:
[8] Witzig A, de Massias de Bonne J, Galletti C, Garin N. Nitrous Oxide-Induced Myelopathy: A Case From French-Speaking Switzerland. Cureus. 2026;18(3):e104698. Published 2026 Mar 5. doi:10.7759/cureus.104698

 

Nitrous oxide myelopathy in an abuser of whipped cream bulbs

Design

  Case report

Case presentation

A 23-year-old man presented with a 5-day history of increasing gait disturbance due primarily to incoordination, with tingling and numbness of the fingers and, to a lesser extent, the feet. He reported nitrous oxide use of approximately 40–60 whipped-cream bulbs daily for 6 months. Examination showed severe proprioceptive impairment in the upper and lower limbs, increased tone and brisk reflexes in the legs, a positive Romberg test, and inability to walk even with support. Laboratory testing showed mild macrocytic anemia and a borderline vitamin B12 concentration of 130 pmol/L; MRI demonstrated increased T2 signal in the posterior cervical cord consistent with posterior column involvement.

He was treated with hydroxycobalamin 1000 mcg intramuscularly daily and oral methionine 1 g three times daily. After 2 weeks, he still required assistance to walk and was discharged to inpatient rehabilitation; at 3-month follow-up, he was walking independently with only minor proprioceptive abnormalities.

Study Author Conclusions

 This case raises awareness of a potentially serious form of abuse of nitrous oxide, which is freely available in the form of bulbs for recharging whipped cream dispensers. The neurological toxicity includes a myelopathy, which can be irreversible or, as in this case, require substantial health resource commitments over a prolonged period. Early recognition is vital and institution of combination therapy with methionine and vitamin B12 is recommended.
Table 5 References:
[9] Butzkueven H, King JO. Nitrous oxide myelopathy in an abuser of whipped cream bulbs. J Clin Neurosci. 2000;7(1):73-75. doi:10.1054/jocn.1998.0149

 

Myeloneuropathy in a dentist

Design

  Case report

Case presentation

A 49-year-old man presented with urinary retention, difficulty ambulating, and a 3-week history of progressive extremity paresthesias, incoordination, and frequent falls. Examination showed inability to ambulate without a walker, profound truncal ataxia, bilateral sustained ankle clonus, a positive Romberg test, and mild weakness of the hands, proximal lower extremities, and feet. He was a dentist who reported occupational nitrous oxide exposure during procedural sedation and denied recreational use. Laboratory testing showed vitamin B12 297 pg/mL, methylmalonic acid 4.67 μmol/L, and homocysteine 45 μmol/L (reference range, 0–13 μmol/L); cervical and thoracic MRI demonstrated T2 hyperintensity involving the posterior columns.

He was treated with vitamin B12 1,000 μg intramuscularly daily for 7 days followed by weekly injections for 1 month and oral L-methionine 500 mg daily. He improved and was discharged after 10 days with outpatient physical therapy; vitamin B12 was 1,275 pg/mL at 2 months, and cervical MRI at 3 months showed complete resolution of the posterior-column T2 hyperintensity. A follow-up homocysteine concentration was not reported.

Study Author Conclusions

 Myeloneuropathy can develop in those individuals who regularly abuse N2O or are chronically exposed in contaminated working environments. Patients often improve with abstinence or removal from the source of exposure; however, supplementation with vitamin B12 and L-methionine may be beneficial.
Table 6 References:
[10] Meyers LE, Judge BS. Myeloneuropathy in a dentist. Clin Toxicol (Phila). 2008;46(10):1095-1096. doi:10.1080/15563650802356617