Understanding Vitamin B12: Low or Borderline Results Explained

Information for patients with a low or borderline (“equivocal”) vitamin B12 result. Bring this with you to talk through the options with your doctor.

Your blood test has shown a vitamin B12 level that is either low or borderline. A borderline (or “indeterminate”) total B12 result usually falls in the range of about 133–258 pmol/L, where deficiency is neither confirmed nor ruled out. When a result is unclear, your doctor may order further tests — such as active B12 (holotranscobalamin), methylmalonic acid or homocysteine — or may suggest a trial of treatment and a repeat test later. This guide explains what B12 is, why it matters, what causes deficiency, and how it is treated.

What is vitamin B12?

Vitamin B12 (also called cobalamin) is a water-soluble vitamin that your body cannot make for itself. You have to get it from food — mainly animal-based foods — or from supplements. Most of your B12 is stored in the liver, and those stores are large enough to last months to years, which is why a deficiency usually develops slowly rather than overnight.

Absorbing B12 from food is a multi-step process. It needs stomach acid to release B12 from food, a protein called intrinsic factor (made by the stomach) to carry it, and a healthy final section of the small bowel (the terminal ileum) to absorb it. This is the key reason that stomach and bowel problems can cause B12 deficiency even when your diet is fine.

What B12 does for your body

Vitamin B12 plays an essential role in cellular metabolism. In particular it is needed for:

• Making DNA and healthy red blood cells. Without enough B12, red cells are produced abnormally — they become large and fewer in number, a pattern called megaloblastic anaemia.
• Keeping your nervous system healthy. B12 helps build and maintain myelin, the protective insulation around nerves. A shortage can damage nerves and, in severe cases, the spinal cord.
• Processing homocysteine, an amino acid in the blood that builds up when B12 (or folate) is low.

What causes B12 deficiency?

There are four broad reasons people become deficient: not enough in the diet, an autoimmune condition, other gut problems, and certain medicines.

1. Not enough B12 in the diet

Because natural B12 comes almost entirely from animal foods, people most at risk include those following a vegan or vegetarian diet, some older adults, people who drink heavily, and people with eating disorders. Vegans and long-term vegetarians have notably higher rates of deficiency, and breastfed infants of deficient mothers are also at risk.

2. Pernicious anaemia and autoimmune gastritis

In this condition the immune system attacks the cells in the stomach that make intrinsic factor. Without intrinsic factor, B12 cannot be absorbed — no matter how much is in your diet. This is one of the most important causes to identify, because it generally requires lifelong injections rather than diet or tablets. If this is suspected, your doctor may test for anti-intrinsic-factor antibodies and look for other autoimmune conditions.

3. Other gut conditions and surgery

Anything that damages the stomach or the lower small bowel can reduce absorption — for example coeliac disease, Crohn’s disease, Helicobacter pylori infection, stomach (gastric) or weight-loss (bariatric) surgery, or removal of part of the ileum.

4. Medicines

• Metformin (for diabetes), used long term, gradually lowers B12 levels.
• Proton pump inhibitors (e.g. esomeprazole, pantoprazole) and H2 blockers (e.g. famotidine), used for more than about two years, can reduce absorption.
• Some other drugs, including colchicine and certain anti-seizure medicines, can also contribute.

Recreational nitrous oxide (“nangs”) inactivates the B12 in your body and can cause deficiency even when blood B12 looks normal.

Why diet alone isn’t always the answer. If your deficiency is caused by an absorption problem (such as pernicious anaemia, coeliac or Crohn’s disease, or after stomach/bowel surgery), eating more B12-rich food — and sometimes even tablets — may not be enough to fix it. In these cases injections are usually needed. Diet-first advice applies when your gut is absorbing B12 normally.

How deficiency can affect you

Symptoms vary widely and often come on gradually. They can include:

• General — tiredness, weakness, reduced appetite, a sore or inflamed tongue and mouth.
• Blood — anaemia: looking pale, breathlessness, a racing heartbeat.
• Nerves — pins and needles, numbness in the hands and feet, and problems with balance.
• Mood & thinking — irritability, low mood, and memory or concentration difficulties.

An important point: nerve and mood symptoms can occur even when your blood count is normal and there is no anaemia, so B12 is sometimes worth checking on its own.

The main risk of leaving it untreated. If a significant deficiency — especially one causing nerve symptoms — is not treated promptly, some nerve and spinal-cord damage can become permanent, even after B12 is replaced. This is why nerve symptoms are treated quickly with injections. In pregnancy, low B12 is also linked with a higher risk of neural tube defects, and infants of deficient mothers can be affected.

How B12 deficiency is treated

The right treatment depends on the cause and the severity, and is decided together with your doctor. The good news is that treatment works well: most people notice an improvement within about two weeks, blood counts usually return to normal within 4–8 weeks, and nerve symptoms tend to improve over roughly 6–12 weeks.

Options can be thought of as a ladder — many people start near the top, but the cause determines how far up you need to go.

1. Food first

Best for — purely dietary deficiency, normal absorption.

If your deficiency is simply due to diet and your gut absorbs B12 normally, eating more B12-rich food may be enough. Natural sources are meat, fish, shellfish, poultry, eggs and dairy. Plant foods do not naturally contain B12, so if you are vegan or vegetarian you’ll need fortified foods (some plant milks, breakfast cereals, nutritional yeast) or a supplement. Most adults need only around 2–2.4 micrograms a day.

2. Oral tablets (high dose)

Best for — dietary and medicine-related causes; an option for many others.

High-dose tablets — typically 1 mg (1,000 micrograms) of cyanocobalamin daily — are an effective, low-cost and convenient option. Clinical trials, including studies in older adults, have shown that high-dose oral B12 can raise blood levels as well as injections do, partly because a small fraction is absorbed without needing intrinsic factor. If tablets are used when absorption may be impaired, your doctor will recheck your blood levels to confirm they are working.

3. Under-the-tongue spray or drops

Best for — people who prefer not to swallow tablets.

A sublingual (under-the-tongue) spray is an alternative to tablets, absorbed through the lining of the mouth and gut. The evidence suggests sublingual and oral routes can be as effective as injections for raising B12 in people without significant absorption problems. Like tablets, a spray will not reliably overcome pernicious anaemia or other absorption problems. See the practical notes below for products available in Australia.

4. Injections into the muscle

Best for — absorption problems, severe deficiency, or nerve symptoms.

In Australia the standard injection is hydroxocobalamin, given into a muscle. A typical pattern is a loading course (for example, 1,000 micrograms on alternate days for 1–2 weeks, then weekly for several weeks), followed by a maintenance dose every 3 months — often lifelong for pernicious anaemia or after relevant stomach/bowel surgery. Injections work quickly and bypass the gut entirely. They are usually given by your nurse or doctor, and some people are taught to give them at home.

When injections are essential. If you have pernicious anaemia or autoimmune gastritis, have had stomach or bowel surgery (including weight-loss surgery), or have nerve symptoms (numbness, tingling, balance problems), tablets and sprays may not be enough — injections are usually needed, and delaying treatment can risk lasting nerve damage. Please do not switch from injections to tablets without your doctor’s advice.

A caution about folate (folic acid). If you are low in B12, avoid taking high-dose folic acid on its own. It can improve the blood (anaemia) picture while allowing nerve damage to quietly progress, which can delay the right diagnosis. B12 and folate should always be sorted out together.

Follow-up and review

If you are taking B12 by mouth, you should be reviewed after about 2–3 months to check your symptoms and that you’re taking it regularly. Your doctor may repeat your B12 (or methylmalonic acid / homocysteine) after 3–6 months to confirm the deficiency has been corrected, particularly if you have an absorption problem. If a reversible cause was found and fixed — such as a medicine that was stopped — treatment may eventually be able to stop; but if your symptoms return, get checked again, as the deficiency may have come back.

Oral B12 sprays in Australia — practical notes

Several under-the-tongue B12 sprays are sold over the counter in Australia, through pharmacies and online retailers. A widely available example is BetterYou Boost B12 oral spray, which delivers about 1,200 micrograms of methylcobalamin per dose (4 sprays), used under the tongue or against the inside of the cheek. Other sublingual sprays and drops combine methyl-, hydroxo- and adenosyl-cobalamin at strengths of roughly 1,000–5,000 micrograms per dose.

These are listed supplements, not prescription medicines, so strength and quality vary between brands — choose a reputable product and tell your doctor what you are taking. The clinical point that oral and sublingual routes can be effective for raising B12 (when absorption is normal) is supported by the trials referenced below.

Please note: This handout is general information and does not replace personalised medical advice. Doses and treatment plans should be confirmed with your own doctor, who can tailor them to your cause of deficiency, your other health conditions and any medicines you take. If you develop new or worsening numbness, tingling, weakness or balance problems, contact your doctor promptly.

References

Every clinical statement in this resource is referenced to a peer-reviewed, PubMed-indexed source or a national guideline. Links open the PubMed record, the article’s permanent (DOI) page, or the guideline.

• Vitamin B12 deficiency: testing and treatment — Bedz D, Forsyth C. Australian Prescriber, 2026;49(2):55–60.
• Clinical practice. Vitamin B12 deficiency — Stabler SP. New England Journal of Medicine, 2013;368(2):149–160.
• Vitamin B12 deficiency — Green R, Allen LH, Bjørke-Monsen AL, et al. Nature Reviews Disease Primers, 2017;3:17040.
• Vitamin B12 deficiency in over 16s: diagnosis and management — National Institute for Health and Care Excellence (NICE). NICE guideline NG239; 2024.
• How prevalent is vitamin B12 deficiency among vegetarians? — Pawlak R, Parrott SJ, Raj S, Cullum-Dugan D, Lucus D. Nutrition Reviews, 2013;71(2):110–117.
• Long term treatment with metformin in patients with type 2 diabetes and risk of vitamin B-12 deficiency: randomised placebo controlled trial — de Jager J, Kooy A, Lehert P, et al. BMJ, 2010;340:c2181.
• Proton pump inhibitor and histamine 2 receptor antagonist use and vitamin B12 deficiency — Lam JR, Schneider JL, Zhao W, Corley DA. JAMA, 2013;310(22):2435–2442.
• Effective treatment of cobalamin deficiency with oral cobalamin — Kuzminski AM, Del Giacco EJ, Allen RH, Stabler SP, Lindenbaum J. Blood, 1998;92(4):1191–1198.
• Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency — Wang H, Li L, Qin LL, Song Y, Vidal-Alaball J, Liu TH. Cochrane Database of Systematic Reviews, 2018;3(3):CD004655.
• Oral versus intramuscular administration of vitamin B12 for vitamin B12 deficiency in primary care: a pragmatic, randomised, non-inferiority clinical trial (OB12) — Sanz-Cuesta T, Escortell-Mayor E, Cura-González I, et al. BMJ Open, 2020;10(8):e033687.
• Oral cyanocobalamin supplementation in older people with vitamin B12 deficiency: a dose-finding trial — Eussen SJ, de Groot LC, Clarke R, et al. Archives of Internal Medicine, 2005;165(10):1167–1172.
• Efficacy of different routes of vitamin B12 supplementation for the treatment of patients with vitamin B12 deficiency: a systematic review and network meta-analysis — Abdelwahab OA, Abdelaziz A, Diab S, et al. Irish Journal of Medical Science, 2024;193(4):1621–1639.
• How I treat cobalamin (vitamin B12) deficiency — Carmel R. Blood, 2008;112(6):2214–2221.
• Neuropsychiatric disorders caused by cobalamin deficiency in the absence of anemia or macrocytosis — Lindenbaum J, Healton EB, Savage DG, et al. New England Journal of Medicine, 1988;318(26):1720–1728.