Left untreated, B12 deficiency can progress from manageable symptoms to serious and in some cases irreversible complications. In the early stages, fatigue, brain fog, and mood changes are the most common complaints. As deficiency deepens, megaloblastic anaemia can develop, where abnormally large red blood cells carry oxygen poorly. Prolonged deficiency can damage the nervous system, causing progressive weakness, balance problems, and cognitive decline that can become permanent if not caught in time. This is why the NHS recommends that anyone at risk, particularly vegans and older adults, take B12 supplementation seriously rather than waiting for symptoms to develop.