Risk factors for a deficiency can include a reduction in intrinsic factor, which is produced in the stomach and is needed for the absorption of vitamin B12.
Practices taken to improve health, such as weight loss surgery and vegetarianism are also known causes of B12 deficiency.
When B12 is stored in the liver, it can sustain the body’s needs for years, so symptoms of a deficiency will generally appear gradually.
The NHS explains: “Vitamin B12 or folate deficiency can cause a wide range of symptoms.