Vitamin B12 deficiency affects multiple systems, including the central and peripheral nervous systems, producing a vast spectrum of neurological symptoms. It is particularly important due to its insidious presentation and because it can evolve to spastic paraplegia with permanent sequelae. The authors describe a case of a woman with asthenia, bilateral lower limb weakness, urinary retention, and faecal incontinence, with no structural cause on imaging studies. Blood tests showed anaemia (haemoglobin: 6.8 g/dL) and vitamin B12 deficiency (<100 pg/mL). After upper digestive endoscopy compatible with chronic atrophic gastritis and positive for anti-intrinsic factor antibodies was obtained, the diagnosis of subacute combined degeneration due to vitamin B12 deficiency in the context of pernicious anaemia was admitted. Although this entity is a rare cause of myelopathy, it is a frequent manifestation of vitamin B12 deficiency. Clinical suspicion is fundamental since the reversibility of the neurological lesion is dependent on early treatment.