Vitamin B12 plays a central role in producing red blood cells, maintaining the myelin sheath that protects nerve fibres, and synthesising DNA. When levels fall, the effects can be wide-ranging and slow-building — which is exactly why B12 deficiency is so often missed.
An estimated 6% of adults under 60 and nearly 20% of those over 60 have B12 deficiency or insufficiency. Symptoms are frequently attributed to ageing, stress or other conditions before the true cause is identified.
Symptoms to recognise
B12 deficiency produces a characteristic cluster of symptoms, though not all appear together:
- **Fatigue and weakness** — the most common complaint, caused by impaired red cell production
- **Numbness or tingling** in the hands, feet or legs — a sign of nerve damage that worsens if untreated
- **Balance and coordination problems** — particularly in older adults
- **Cognitive symptoms** — memory difficulties, brain fog, difficulty concentrating
- **Glossitis** — a sore, smooth, inflamed tongue
- **Pallor or jaundice** — from macrocytic anaemia (enlarged, poorly functioning red cells)
- **Mood changes** — depression and irritability
Who is at risk
- People aged over 60 — stomach acid production declines with age, reducing B12 absorption
- People taking metformin for diabetes — metformin reduces B12 absorption over time
- People on long-term proton pump inhibitors (PPIs)
- Strict vegans and vegetarians — B12 is found almost exclusively in animal products
- People with autoimmune pernicious anaemia — the immune system attacks the cells that produce intrinsic factor, which B12 needs for absorption
- Anyone who has had gastric surgery or has inflammatory bowel disease
What the blood test measures
Serum B12 is the standard screening test. Normal ranges vary slightly by laboratory, but results below 148 pmol/L (200 pg/mL) are generally considered deficient, and those between 148–221 pmol/L are borderline and warrant further investigation.
Serum B12 has limitations. Total B12 includes inactive forms that don't reach tissues, so a normal total level doesn't always rule out functional deficiency. If clinical suspicion is high, doctors often add methylmalonic acid (MMA) and homocysteine — both of which rise early in B12 deficiency, before the serum level falls.
Treatment
For most people, oral B12 supplementation at high doses (1,000 mcg daily) is as effective as injections, provided the gut can absorb it. For pernicious anaemia — where intrinsic factor is absent — injections bypass the absorption problem and are the standard treatment.
Neurological symptoms respond slowly and sometimes incompletely if deficiency has been present for a long time. This is why early diagnosis matters: the sooner it's caught, the more fully reversible the damage.