What are the symptoms of vitamin B12 deficiency?

Tiredness is the symptom most associated with B12 deficiency, and on its own it is close to useless as a signal, because almost everything causes tiredness. The more distinctive features described by the NHS include pins and needles, a sore or red tongue, mouth ulcers, muscle weakness, and disturbances of vision, memory or mood.

The neurological symptoms are the reason it is worth taking seriously rather than treating as a vitamin question. Some can persist if a deficiency goes unaddressed for a long time, which is why the advice is to get it tested rather than start supplementing on the assumption.

Testing matters for another reason: supplementing before a test can obscure the result and hide the underlying cause, which may be dietary, absorption-related or something else. That is a sequence question for a clinician.