How do you know if your vitamin B12 levels are low?

In this article, we discuss:

• What is a vitamin B12 deficiency?
• What are the symptoms of a B12 deficiency
• Who is at risk of a vitamin B12 deficiency
• How is B12 deficiency diagnosed
• What is a low vitamin B12 result?
• How is B12 deficiency treated

What is vitamin B12 deficiency?
 

The human body stores several years' worth of vitamin B12 in the liver, so it can often manage these reserves if it is not provided with B12 from an external source. However, eventually, these reserves will run out too and can lead to a deficiency. You can read more about why we need vitamin B12 in our blog.

Vitamin B12 deficiency occurs when the body's stores become depleted. This can be due to insufficient intake, poor absorption or, more rarely, increased requirements or use.

This could be due to a mixture of things, including:

• Lack of vitamin B12 in the diet
• Problems absorbing the vitamin from the gut
• Autoimmune conditions

What are the symptoms of B12 deficiency?
 

As vitamin B12 is necessary for so many functions in the body, the symptoms of a deficiency are wide-ranging.

Symptoms of a B12 deficiency typically include:

• Fatigue
• Breathlessness
• Loss of appetite
• Digestive issues
• Weight loss
• Mouth and tongue ulcers
• Poor balance
• Brain fog
• Pins and needles or loss of sensation in fingers and toes
• Muscle weakness
• Sight and memory problems
• Mood changes and depression
• Palpitations

You should speak to a medical professional or test yourself for a B12 deficiency if you are experiencing any of these symptoms, as a long-term deficiency can be detrimental to your health. So, the earlier it’s detected, the better.

Potential complications of a B12 deficiency include:

• Anaemia
• Permanent nerve damage
• Difficulty walking
• Pregnancy complications

However, a deficiency of vitamin B12 can easily be detected through a blood test, and in most cases, can be easily reversed.

What causes vitamin B12 deficiency, and who is at risk?
 

The most common cause of vitamin B12 deficiency is an autoimmune disease called pernicious anaemia. This condition causes the body to produce an .antibody that attacks intrinsic factor, or the stomach cells that produce it, preventing vitamin B12 from being absorbed in the small intestine.

Other people at higher risk of a vitamin B12 deficiency include people:

• With a gut autoimmune disease – such as Crohn’s disease or coeliac. This is because the body attacks the gut and can affect the gut lining, meaning that B12 is not absorbed properly.
   
• On metformin – people who are on metformin are at greater risk of developing a vitamin B12 deficiency. The government advises you are screened for a vitamin B12 deficiency if you are on long-term metformin [2].
   
• Over the age of 60 – it is thought that up to one in five people over the age of 60 may be affected by a B12 deficiency due to the decline in absorption rates as you get older [3].
   
• Who are vegan or vegetarian – people who don’t consume any animal products are at risk as this is one of the main sources of vitamin B12.
   
• Who overuse nitrous oxide – also known as laughing gas, this anaesthetic gas is also a popular party drug which, when overused, converts active vitamin B12 into its inactive form [4,5]

How is vitamin B12 deficiency diagnosed?
 

A vitamin B12 deficiency is diagnosed through a simple blood test, and to help you get the answers you need, Medichecks has a range of vitamin B12 blood tests and a Pernicious Anaemia Blood Test.

What is a low vitamin B12 level?
 

When people talk about having low vitamin B12, they’re usually referring to a blood test result. In the UK, doctors use guidance from the National Institute for Health and Care Excellence (NICE) to decide whether a level is normal, borderline, or low enough to count as a deficiency.

The table below highlights the cut-offs:

Active vitamin B12 reflects the amount immediately available for cellular use, and may identify deficiency earlier than total vitamin B12 in some people.

For example, even if the results showed a normal range of vitamin B12 in the blood, the levels of useful active B12 could still be low. This is why Medichecks measures active B12 providing clear, accurate, and actionable results.

Take a look in our blog about the difference between active and total vitamin B12 for more information.

How is a vitamin B12 deficiency treated?
 

A vitamin B12 deficiency or pernicious anaemia should be diagnosed and treated promptly – some problems caused by the condition can be irreversible, though complications are rare [6]. 

Once a blood test has diagnosed a vitamin B12 deficiency, the first step is to look at your diet and see if adjustments can be made to increase your intake. Supplements and tablets are useful unless you have a gut absorption condition, in which case injections are required.

What’s the recommended intake of vitamin B12?

The recommended daily amount of vitamin B12 for adults aged 19-64 is 1.5 micrograms a day.

In some cases, including more vitamin B12 in your diet can treat the deficiency and prevent its return. However, if your intake still isn’t high enough, you may need to take daily tablets in between meals or have regular injections which may need to be continued for the rest of your life.

References

1. The difference between B12 Deficiency and Pernicious Anaemia. In: https://pernicious-anaemia-society.org/ [Internet]. [cited 26 Jun 2026]. 
2. Metformin and reduced vitamin B12 levels: new advice for monitoring patients at risk. In: GOV.UK [Internet]. [cited 26 Jun 2026]. 
3. Prevalence | Background information | Anaemia - B12 and folate deficiency | CKS | NICE. [cited 11 Sep 2025]. 
4. Anaemia - B12 and folate deficiency | Health topics A to Z | CKS | NICE. [cited 25 Jun 2026]. 
5. Nadal Bosch J, Malcolm J, Moya M, Menowsky M, Cruz RA. A Case Report of Subacute Combined Degeneration Due to Nitrous Oxide-Induced Vitamin B12 Deficiency. Cureus. 2023;15: e34514.
6. Jatoi S, Hafeez A, Riaz SU, Ali A, Ghauri MI, Zehra M. Low Vitamin B12 Levels: An Underestimated Cause Of Minimal Cognitive Impairment And Dementia. Cureus. 2020 Feb 13;12(2):e6976. doi: 10.7759/cureus.6976. PMID: 32206454; PMCID: PMC7077099.
7. Vitamins and minerals - B vitamins and folic acid. In: nhs.uk [Internet]. 23 Oct 2017 [cited 26 Jun 2026].