Vitamin B12 or B9 (commonly called folate) deficiency anaemia occurs when a lack of vitamin B12 or folate causes the body to produce abnormally large red blood cells that cannot function properly.

Red blood cells carry oxygen around the body using a substance called haemoglobin.

Anaemia is the general term for having either fewer red blood cells than normal or having an abnormally low amount of haemoglobin in each red blood cell.

Vitamin B12 and folate perform several important functions in the body, including keeping the nervous system healthy.

A deficiency in either of these vitamins can cause a wide range of problems, including:

• extreme tiredness
• a lack of energy
• pins and needles
• a sore and red tongue
• mouth ulcers
• muscle weakness
• problems with your vision
• psychological problems, which can range from mild depression or anxiety to confusion and dementia
• problems with memory, understanding and judgement

Some of these problems can also happen if you have a deficiency in vitamin B12 or folate but do not have anaemia.

See a GP if you think you may have a vitamin B12 or folate deficiency.

These conditions can often be diagnosed based on your symptoms and the results of a blood test.

It's important for vitamin B12 or folate deficiency anaemia to be diagnosed and treated as soon as possible.

This is because although many of the symptoms improve with treatment, some problems caused by the condition can be irreversible.

There are a number of problems that can lead to a vitamin B12 or folate deficiency.

These include:

• pernicious anaemia – where your immune system attacks healthy cells in your stomach, preventing your body absorbing vitamin B12 from the food you eat; this is the most common cause of vitamin B12 deficiency in the UK
• a lack of these vitamins in your diet – this is uncommon, but can happen if you have a vegan diet and do not take vitamin B12 supplements or eat foods fortified with vitamin B12 , follow a restrictive diet or have a generally poor diet for a long time
• medicine – certain medicines, including anticonvulsants and proton pump inhibitors (PPIs), can affect how much of these vitamins your body absorbs

Both vitamin B12 deficiency and folate deficiency are more common in older people, affecting around 1 in 10 people aged 75 or over and 1 in 20 people aged 65 to 74.

Most cases of vitamin B12 and folate deficiency can be easily treated with injections or tablets to replace the missing vitamins.

Vitamin B12 supplements are usually given by injection at first.

Then, depending on whether your B12 deficiency is related to your diet, you'll either require B12 tablets between meals or regular injections.

Treatments may last until your vitamin B12 levels have improved or you may need treatment for the rest of your life.

Folic acid tablets are used to restore folate levels. These usually need to be taken for 4 months.

In some cases, improving your diet can help treat the condition and prevent it coming back.

Vitamin B12 is found in meat, fish, eggs, dairy products and specially fortified foods.

Good sources of folate include green vegetables, such as broccoli, brussels sprouts and peas.

Although it's uncommon, vitamin B12 or folate deficiency (with or without anaemia) can lead to complications, particularly if you have been deficient in vitamin B12 or folate for some time.

Potential complications can include:

• problems with the nervous system
• temporary infertility
• heart conditions
• pregnancy complications and birth defects

Adults with severe anaemia are also at risk of developing heart failure.

Some complications improve with appropriate treatment, but others, such as problems with the nervous system, can sometimes be permanent.