Vitamin B12: deficiency, symptoms and supplements explained

Vitamin B12 comes mainly from animal foods like meat, fish, eggs and dairy. If you eat enough of these you’ll usually get plenty, but if you eat little or no animal produce, you’re older, or you have certain gut or medication issues, a deficiency is more likely and a supplement may help. It’s one of those vitamins you tend to notice only when you feel tired and flat, yet it quietly runs a lot in the background: your energy, your nervous system and the making of red blood cells all rely on it. Below we explain what B12 does, how to spot a deficiency, who’s most at risk, and how to pick between the different forms and doses.

What vitamin B12 does in your body

Vitamin B12 (also called cobalamin) is involved in three things you need every single day.

• Turning food into energy. B12 helps your body convert nutrients into usable energy. When it runs low, one of the first things you tend to feel is tired and sluggish.
• Keeping your nervous system healthy. B12 helps maintain the protective layer (myelin) around your nerves. If that breaks down, you can get tingling or numbness in your hands and feet.
• Making red blood cells. Together with folate, B12 helps produce the red blood cells that carry oxygen around your body. A shortfall can lead to anaemia.

So B12 works away in the background, but when it’s missing you tend to feel it on several fronts at once.

Symptoms of a B12 deficiency

A deficiency usually builds slowly. Your liver holds a store that can last months or even years, so symptoms tend to creep in rather than arrive suddenly, which also makes them hard to pin on B12.

Common signs include:

• Ongoing tiredness and low energy
• Tingling, numbness or a pins-and-needles feeling in your hands and feet
• Paleness and breathlessness (from anaemia)
• Trouble concentrating, forgetfulness or a low mood
• A sore or burning tongue
• Muscle weakness or an unsteady walk

These symptoms fit a lot of different causes, so on their own they don’t prove a B12 deficiency. If you recognise several at once, especially if you fall into a higher-risk group, get it checked. Left untreated, a long-term deficiency can cause lasting nerve damage, so it isn’t something to sit on.

Who is most at risk?

Not everyone has the same odds of running low. A few groups stand out.

• Vegans and vegetarians. B12 naturally comes almost entirely from animal foods. If you don’t eat meat, fish, eggs or dairy, you get very little from your diet. For vegans a supplement (or fortified foods) is realistically always needed.
• Older adults. From around age 50 the stomach often makes less acid, so you absorb B12 from food less well even if you’re eating plenty.
• People with gut conditions. Conditions like Crohn’s disease, coeliac disease or stomach surgery can interfere with absorption.
• People on certain medications. Long-term use of metformin (for type 2 diabetes) and acid-reducing medicines (PPIs) can lower how much B12 you take in.
• People with pernicious anaemia. Here the body doesn’t make enough intrinsic factor, a substance needed to absorb B12.

If any of these apply to you, it’s sensible to keep an eye on your B12, even when you feel fine.

Food sources of vitamin B12

B12 is made by bacteria and reaches our food mainly through animal produce. Good sources include:

• Meat, particularly organ meat such as liver
• Fish and shellfish
• Eggs
• Milk, cheese and other dairy

Plant foods contain virtually no reliable B12 in their natural state. Algae, seaweed or fermented foods are sometimes mentioned, but they don’t provide a form your body can properly use, so you can’t count on them. If you eat plant-based, there are two dependable routes: fortified foods (some plant milks and breakfast cereals have B12 added) or a supplement. For most vegans a supplement is the surest choice, because fortified foods don’t always deliver enough, or deliver it consistently.

How much B12 do you need a day?

The daily need for B12 is small. For adults the recommended amount is roughly a few micrograms a day, with a little more during pregnancy and breastfeeding. Worth knowing: your body can only absorb a limited amount of B12 at a time, and the bigger the single dose, the smaller the percentage you actually take up. That’s why supplements often contain far more than those few micrograms. A tablet with, say, 10 to 25 micrograms is completely normal, because you only absorb part of it, you still end up with enough.

With some absorption problems, such as in older adults or with pernicious anaemia, much higher doses are used (hundreds up to a thousand micrograms), or injections arranged through your GP. That’s tailored care you work out with a doctor, not something to guess at yourself.

Doses and forms: methylcobalamin or cyanocobalamin?

In supplements you’ll mostly come across two forms: cyanocobalamin and methylcobalamin. The difference sounds technical, but it boils down to this.

• Cyanocobalamin is the most widely used and best-studied form. It’s stable, cheap, and your body converts it into the active form itself. For most people this works perfectly well.
• Methylcobalamin is an already-active form your body can use directly. It’s often marketed as more natural, but the evidence that it clearly works better in practice than cyanocobalamin is limited.

For most people the choice between the two makes little difference. What matters more is that you get enough in the first place and take it regularly. In practice both forms are inexpensive, often just a few pounds for a month or two, so cost isn’t much of a deciding factor. Take a supplement whichever way is easiest for you, daily or a few times a week, depending on the dose on the packet, and follow the label if you’re unsure.

Absorption: the role of intrinsic factor

Absorbing B12 is more involved than simply swallowing it. Your stomach needs acid to release B12 from food, and the B12 then binds to a protein called intrinsic factor. That protein is made in your stomach and works like a key: without it, your gut can’t take the B12 up properly.

This is exactly why some people end up deficient despite eating plenty of B12. If you make little stomach acid (often with age) or little intrinsic factor (as in pernicious anaemia), absorption falls short. Supplements can partly get around this, because a small amount of B12 is also absorbed passively without intrinsic factor. But with a genuine absorption problem, higher doses or injections are sometimes needed. That’s why, with ongoing symptoms, it’s better to have the cause investigated rather than endlessly self-treating.

Can you get too much B12?

Good news: B12 is water-soluble, which means your body largely passes any excess in your urine rather than storing it. No upper limit has been set for B12, because harmful effects in healthy people haven’t been shown, even at higher doses. That makes it hard to overdo it through food or normal supplements. Still, more isn’t automatically better, above what you need you simply pass it out, so extra-high doses give no added benefit if you’re not deficient.

One exception: an unusually high B12 reading in your blood when you’re not taking supplements can occasionally point to an underlying issue. That’s something your doctor assesses, not a reason to panic, but worth discussing.

How a deficiency is diagnosed

You can’t diagnose a B12 deficiency on how you feel or on symptoms alone, because they overlap with too many other things. The reliable way is a blood test through your GP. Alongside the B12 level itself, a doctor sometimes looks at extra markers (such as active B12 or certain by-products in your blood), because the standard reading doesn’t always show the full picture. From there the doctor decides whether there’s a real deficiency and what works best: adjusting your diet, a supplement at the right dose, or in some cases injections.

In short: if you suspect a deficiency, especially if you’re in a higher-risk group or have several symptoms, don’t start blindly taking high doses. Get it measured. That way you know where you stand and you avoid missing another cause. This guide helps you understand what’s going on, but the diagnosis belongs with your GP. Nothing here replaces medical advice.