Written by Dr. Simon Khela MBChB MRCGP, GMC Registered Doctor
Last reviewed: 27-07-2026
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Vitamin B12 deficiency doesn't always look the way people expect. It can be subtle, it's easily missed on a standard blood count, and left untreated for long enough, some of the neurological effects don't fully reverse. It's also, in most cases, straightforward to diagnose and treat once it's actually looked for.
This article covers:
Vitamin B12 deficiency happens when the body doesn't have enough cobalamin to make healthy red blood cells and maintain normal nerve function. It's usually caused by one of two things: not getting enough B12 from diet (most relevant for vegans and some vegetarians), or the body being unable to absorb it properly, the more common cause in the UK, including conditions such as pernicious anaemia.
Vitamin B12 (cobalamin) is a water-soluble vitamin the body needs for two main jobs: making healthy red blood cells and keeping the nervous system working properly. Unlike vitamin D, the body can't make B12 itself, so every bit has to come from diet or supplementation.
Many patients are surprised to learn how wide-ranging the symptoms of B12 deficiency can be, and how easily they get attributed to something else, particularly stress, ageing, or being generally run down.
One of the most common concerns I see in practice is a patient, often over 60, worried about worsening memory who turns out to have a straightforward and treatable B12 deficiency rather than a neurodegenerative condition. It’s a good example of why a proper work-up matters before jumping to more worrying conclusions, and it’s one reason a memory assessment should generally include basic bloods, including B12 and folate, before considering anything more specialist.
Importantly, neurological symptoms can appear before anaemia shows up on a standard blood count, particularly in people who are also taking folic acid supplements, which can mask the blood picture of B12 deficiency while the nerve damage continues underneath. This is precisely why B12 should be checked specifically, not assumed to be fine just because a full blood count looks normal.
Vitamin B12 deficiency isn't only a concern for vegans, though that's often the first group people think of. In UK general practice, the more common causes relate to absorption rather than diet alone.
Since B12 occurs naturally only in animal products, anyone avoiding meat, fish, eggs, and dairy is at meaningfully increased risk without deliberate supplementation or fortified foods.
Stomach acid production naturally declines with age, and a condition called atrophic gastritis becomes more common, both of which reduce the release and absorption of B12 from food, even when dietary intake looks perfectly adequate.
This autoimmune condition, where the body attacks the cells that produce intrinsic factor, is one of the most common causes of B12 deficiency I see in clinic and requires lifelong treatment, usually injections, since oral absorption remains impaired regardless of diet.
Crohn's disease, coeliac disease, or anyone who has had bowel or gastric surgery, including weight-loss surgery, can significantly reduce the gut's ability to absorb B12.
Metformin (commonly prescribed for type 2 diabetes) and long-term proton pump inhibitors (PPIs) used for acid reflux are both well recognised to reduce B12 absorption over time. Anyone on long-term metformin having ongoing diabetes monitoring should have B12 checked periodically alongside their usual reviews, something that isn’t always routinely flagged.
This is an area I've seen come up more often in recent years. Nitrous oxide inactivates vitamin B12 in the body, and regular or heavy recreational use can cause a rapid, sometimes severe functional deficiency with significant neurological symptoms, even in someone with otherwise normal dietary intake and blood levels beforehand.
Requirements increase during pregnancy and breastfeeding, particularly for those following a vegan or restricted diet, since the baby depends entirely on maternal stores and intake.
Diagnosis starts with a blood test measuring serum B12 levels, usually as part of a broader panel that also checks folate, since the two deficiencies often overlap and interact.
A straightforward serum B12 result isn't always the full picture, though. Total B12 blood levels can be falsely reassuring or falsely low in certain circumstances, which is why current UK guidance (NICE's 2024 guideline on vitamin B12 deficiency, NG239) supports using additional markers where the diagnosis is unclear, particularly:
In practice, for most patients, a well-timed private blood test covering B12, folate, and a full blood count, interpreted alongside symptoms and risk factors, gives a clear enough answer to start treatment. More detailed testing tends to be reserved for cases where the diagnosis is borderline or the cause isn’t obvious.
If pernicious anaemia is suspected, or if there's a family history of autoimmune conditions such as thyroid disease or vitiligo (which cluster together), it's also worth discussing this with your GP, since confirmed autoimmune gastritis requires longer-term monitoring, including a small increased risk of stomach cancer that NICE guidance specifically flags for follow-up.
This is one of the most common questions I'm asked, and it comes with a fact that often surprises patients: vitamin B12 occurs naturally only in animal-based foods. There is no meaningful natural source in fruit or vegetables, however it's often assumed otherwise.
The UK Reference Nutrient Intake for adults is 1.5 micrograms of vitamin B12 a day, a relatively small amount that's easily met through a varied diet containing meat, fish, eggs or dairy. The challenge arises for anyone who avoids all of these, or whose body can't absorb what they're eating regardless of intake.
I want to address this directly, because it's a genuine and common point of confusion. Vegetables and fruits do not contain meaningful, reliable amounts of vitamin B12. Some plant foods, such as certain seaweeds, mushrooms, and fermented products like tempeh, have occasionally been promoted as plant-based B12 sources, but the evidence doesn't support relying on them. Many contain B12 analogues, inactive compounds that resemble B12 but don't function the same way in the body, and in some cases may even interfere with the absorption of genuine B12.
For anyone following a vegan diet, the practical, evidence-based approach is fortified foods (checked specifically for added B12 on the label) or a B12 supplement, rather than assuming a plant-rich diet will provide adequate levels on its own. This is a point the Vegan Society is itself very clear and consistent about, which says a great deal, given it's an organisation with every reason to want the answer to be different.
For most people without an absorption problem, the most reliable way to maintain healthy B12 levels is through regular dietary intake rather than supplementation, though supplements have an important role for specific groups.
Patients often ask whether there's an ideal time of day to take a B12 supplement, and honestly, the evidence for a strict “best time” is limited. That said, a few practical points are worth knowing.
If you're taking multiple supplements, spacing them out isn't generally necessary for B12 specifically, though it's always worth checking with a pharmacist or GP if you're on multiple medications or supplements together, particularly if you're also taking calcium or medication for acid reflux, both of which can modestly affect absorption over the long term.
Treatment depends heavily on the underlying cause, which is why an accurate diagnosis matters before starting anything.
For confirmed deficiency, particularly where malabsorption or pernicious anaemia is the cause, vitamin B12 injections (hydroxocobalamin) are typically the first-line treatment. A loading course, often several injections over a couple of weeks, corrects levels quickly, especially where neurological symptoms are present, followed by maintenance injections every two to three months, sometimes for life, if the underlying cause (such as pernicious anaemia) isn’t going to resolve on its own.
For dietary deficiency without an absorption problem, high-dose oral B12 tablets can be effective and are increasingly used even in some cases of mild pernicious anaemia, since a small proportion of oral B12 can still be absorbed passively without intrinsic factor. Whether oral treatment or injections are more appropriate depends on the cause and severity, which is a conversation worth having directly with your GP rather than assuming one option over the other.
Some patients ask about IV vitamin drip therapy as an alternative route. It can deliver B12 alongside other nutrients in a single session, though for straightforward B12 deficiency alone, standard injections remain the well-established, evidence-based approach, and IV therapy tends to suit people looking for a broader nutrient top-up rather than a targeted deficiency treatment.
Most patients notice improved energy within a few weeks of starting treatment, though established neurological symptoms, particularly numbness or balance problems that have been present for months, can take longer to improve and don't always fully resolve if treatment was significantly delayed. This is really the core message I try to get across: earlier treatment gives a better outcome, which is why it's worth investigating persistent symptoms rather than waiting.
As covered above, this isn't accurate. Reliable natural sources are animal-based; plant-based diets need fortified foods or supplements.
Not necessarily. Neurological symptoms can appear before anaemia does, particularly if folate levels are adequate or supplemented, which can mask the usual blood picture.
In UK general practice, absorption problems, including pernicious anaemia, ageing-related changes, and certain medications, are at least as common a cause as diet, often more so.
There's no good evidence that supplementing B12 improves energy or wellbeing in someone who isn't actually deficient. If levels are already adequate, extra B12 is simply excreted rather than providing additional benefit.
Pernicious anaemia is a problem of absorption, not intake. Diet alone won't correct it; treatment needs to bypass or work around the impaired absorption pathway, usually through injections or high-dose oral tablets.
Testing for B12 deficiency is a simple blood test, low-risk, and widely available, and correcting a genuine deficiency is generally safe and highly effective, often producing a noticeable improvement in energy, mood, and neurological symptoms.
It's worth being honest about the limitations too. Not everyone with fatigue or low mood has a B12 deficiency as the cause, and treating B12 levels shouldn't replace investigating other possible explanations if symptoms don't improve. Interpreting borderline results can also be genuinely difficult, since total B12 levels don't always correlate perfectly with how the body is actually functioning at a cellular level, which is part of why additional markers like MMA exist. And while B12 treatment is very safe, with side effects from injections being uncommon and generally mild, it isn't a general-purpose energy booster for people who aren't actually deficient, whatever's sometimes suggested online.
I’d generally suggest arranging a blood test, alongside a proper clinical assessment, if you have:
An online GP consultation is often a practical first step if you'd like to discuss symptoms and arrange the right blood tests without waiting for a routine appointment slot. If the cause turns out to be more complex, for example unexplained malabsorption without a clear diagnosis, a referral to a specialist such as a gastroenterologist may be appropriate to investigate further.
It's also worth flagging proactively if you're perimenopausal or menopausal and experiencing fatigue, low mood, or brain fog, since these overlap significantly with both hormonal changes and B12 deficiency, and it's easy for one to be assumed when the other, or both, are contributing. This is something I'd routinely check alongside other bloods for anyone attending a menopause clinic with these symptoms.
Early signs often include persistent fatigue, pale skin, mild breathlessness, and a sore or unusually smooth tongue. Some people also notice early tingling in the hands or feet, which shouldn’t be ignored even if other symptoms feel mild.
Yes, in most cases, particularly if caught reasonably early. Blood-related symptoms such as anaemia usually improve within weeks of treatment. Neurological symptoms present for a long time before diagnosis may improve more slowly and, in some cases, not fully resolve.
Shellfish (particularly clams and mussels), liver, oily fish such as mackerel and salmon, and to a lesser extent eggs, milk and cheese, are among the richest natural sources. Fortified breakfast cereals and plant milks are useful additional sources, particularly for vegetarians and vegans.
No, not in any reliable or meaningful amount. B12 occurs naturally only in animal-based foods, so anyone on a vegan diet needs fortified foods or a supplement rather than relying on fruit and vegetables.
There’s no strong evidence for one specific “best” time. Taking it consistently, often in the morning as part of an existing routine, matters more than the exact time of day.
Mood changes, including low mood and irritability, are recognised symptoms of B12 deficiency, though B12 deficiency is rarely the sole cause of anxiety or depression and shouldn’t be assumed to be without a wider assessment.
Through a blood test measuring serum B12 levels, usually alongside folate and a full blood count. Where the result is unclear, additional tests such as methylmalonic acid or intrinsic factor antibodies may be used.
Vitamin B12 is water-soluble, and excess is generally excreted in urine, so it's considered to have a low risk of toxicity even at higher supplement doses. That said, taking it without a confirmed need doesn't provide additional benefit either.
Injections bypass the gut absorption process entirely, which matters for people whose deficiency is caused by poor absorption, such as pernicious anaemia, rather than low dietary intake. For dietary deficiency alone, oral tablets are often sufficient.
Long-term metformin use is a well-recognised cause of reduced B12 absorption, and periodic monitoring is worth discussing with a GP if you've been on it for several years, even without obvious symptoms.
Yes, particularly in older adults, where it can cause confusion or memory problems that are sometimes mistaken for dementia. This is one of several reasons B12 and folate are usually checked as part of a proper memory assessment.
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