Important reminder: nitrous oxide (N2O) as a cause of functional vitamin B12 deficiency.

A 32-year-old woman who misused multiple substances, including nitrous oxide (N2O), sought medical advice after she subacutely developed bilateral lower extremity weakness without a sensory level but with ataxia-her significant other developed similar symptoms with vitamin B12 deficiency due to N2O intake. Laboratory results revealed macrocytic anaemia despite normal B12 and folate levels, with serum markers pointing towards functional cobalamin deficiency. Spinal MRIs and cerebrospinal fluid analysis were unremarkable. Our patient was treated with vitamin B12 supplementation with an encouraging response.