Abstract
A 32-year-old woman who misused multiple substances, including nitrous oxide (N(2)O), 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 B(12) deficiency due to N(2)O intake. Laboratory results revealed macrocytic anaemia despite normal B(12) 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 B(12) supplementation with an encouraging response.