A vitamin B12 deficiency is treacherous. It develops slowly, the symptoms are vague, and precisely because of that it can go unnoticed for years. You blame fatigue on stress, tingling on bad posture, while a prolonged deficiency can damage your nervous system.
My view: with tingling plus fatigue, do not wait and see. Early nerve damage is often still reversible, late damage sometimes is not.
What does vitamin B12 do?
Vitamin B12 (cobalamin) is needed to make red blood cells, for the functioning of your nervous system (protecting the myelin sheath), for DNA synthesis and cell division, and to make neurotransmitters that affect your mood and cognition. It occurs almost exclusively in animal products, which immediately explains why certain groups are at extra risk.
Vitamin B12 deficiency symptoms
The symptoms often develop gradually. The combination of fatigue with tingling fits a B12 deficiency better than fatigue alone, which can also point to a vitamin D deficiency. Common complaints:
- Fatigue and weakness, from reduced red blood cell production
- Tingling or numbness in hands and feet, from nerve damage
- Trouble concentrating and forgetfulness
- Low mood or irritability
- Balance problems, from damage to the nervous system
- A painful, red tongue (glossitis)
- Paleness or a yellowish skin
The nerve damage from a prolonged B12 deficiency can be irreversible. Early detection is therefore important.
Vitamin B12 reference values
The value is measured in pmol/L. The cut-offs differ slightly per laboratory, but as a guide:
| Value (pmol/L) | Interpretation | Logical step |
|---|---|---|
| Above 300 | Normal | No action if no symptoms |
| 150 to 300 | Grey zone, possible functional deficiency | Additional MMA or homocysteine |
| Below 150 | Deficiency | Discuss supplementation with your doctor |
For values in the grey zone, an additional test for methylmalonic acid (MMA) or homocysteine can clarify a functional deficiency, because these substances rise when the B12 inside your cells falls short.
Why a normal value can still be too low
A tricky point: the standard B12 assay measures all B12 in your blood, including the form your cells cannot use. As a result your value can fall just within the range while your cells still fall short. This is called a functional deficiency. The NHG guideline for GPs therefore advises looking at MMA when values are doubtful or symptoms are clear, because that substance rises specifically with a real B12 deficiency at cell level. This prevents you from missing a deficiency simply because the number looks reassuring.
B12 from food
Vitamin B12 occurs almost exclusively in animal products: meat, fish, eggs and dairy. Plant sources provide no usable B12 in practice, not even algae or fermented products that are sometimes claimed to. The Voedingscentrum therefore advises vegans and people who eat virtually no animal products to take a B12 supplement or fortified products as standard. If you do eat animal products but little, your intake is usually enough as long as your absorption works well.
Who is at risk of a vitamin B12 deficiency?
- Vegetarians and vegans, because B12 is found almost exclusively in animal products
- Older adults (65+), in whom absorption of B12 from food declines
- People with gastrointestinal problems such as coeliac disease, Crohn's or after stomach surgery
- Users of acid reducers, because long-term PPI use lowers absorption
- Metformin users, because this diabetes medicine can reduce B12 absorption
- People with pernicious anaemia, an autoimmune condition that blocks absorption
Supplementing vitamin B12
With a proven deficiency, the approach depends on the cause and severity. For mild deficiencies oral supplements often suffice, while for absorption problems or severe deficiencies your doctor may prescribe injections. Discuss the right form and dose with your GP. Read more in supplementing vitamin B12 with tablets or injections.
An important distinction: whether the deficiency comes from too little intake or from an absorption problem. In a vegan without supplements, intake is the issue, and a tablet usually works well. In someone with pernicious anaemia or after stomach surgery, absorption itself falters, and then high oral doses or injections are needed. That is why measuring your value alone is not enough: the cause determines the treatment, and you work that out together with your doctor.
Frequently asked questions
How long does it take to correct a B12 deficiency?
Improvement in your blood values is often visible within 6 to 8 weeks. Nerve complaints can take months to recover and, with a prolonged deficiency, are sometimes not fully reversible.
Can I take too much vitamin B12?
Vitamin B12 is water-soluble: an excess is excreted in your urine. Toxicity from supplements is barely described. Still, it is wise to supplement based on a proven deficiency.
Do I need to fast for a B12 test?
No, fasting is not needed. Do always mention whether you use B12 supplements, because that affects the result.
Getting your vitamin B12 measured
If you want your B12 checked, view your vitamin B12 value. Because fatigue fits several deficiencies, the B-vitamins panel also covers folate, while the tiredness panel looks more broadly at the causes of fatigue. This article is part of our vitamin deficiency overview and connects to folate deficiency.
Every blood test result at Vitalcheck includes a professional assessment by a BIG-registered doctor. A blood value is not a diagnosis: always discuss symptoms and treatment decisions with your GP.
Sources
- Thuisarts.nl / NHG. Vitamin B12: what if you have too little? Accessed 2026.
- Gezondheidsraad. Dietary reference values for vitamins and minerals. 2018.
- Voedingscentrum. Vitamin B12. Accessed 2026.
Tags
Author
