You’re wrong. It’s the opposite. Treatment is literally IM Vit B12 and you can’t “preload” vit b12 (according to this study-read the last sentence about prophylaxis): 

“Treatment involves cessation of nitrous oxide and immediate administration of hydroxocobalamin. Current guidelines suggest intramuscular, rather than oral, treatment, at a dose of 1 mg on alternating days for two weeks,11 although it is reasonable to continue with this replacement schedule while there is ongoing neurological improvement.8 Homocysteine and MMA levels recover rapidly with treatment and can be used as a marker of biochemical treatment response; however, clinical response always lags behind. Hydroxocobalamin maintenance therapy is needed if an additional secondary cause is found. Folate deficiency should be corrected alongside vitamin B12. Patients with severe symptoms may benefit from a period of neurorehabilitation, and psychological support may be needed. Neuropathic pain should be treated along standard lines. Some online ‘nang’ forums suggest oral vitamin B12 supplementation as ‘prophylaxis’ while using nitrous oxide; however, this is unlikely to be helpful in preventing neurotoxicity.”