This guideline covers the diagnosis and management of vitamin B12 deficiency, including deficiency caused by autoimmune gastritis. The guideline includes recommendations on how to identify, test and treat vitamin B12 deficiency based on its cause and monitoring for complications. It aims to raise awareness of the condition in people aged 16 and over, including those who are pregnant or breastfeeding, leading to better diagnosis, treatment and ongoing care. We have used specific inclusive language (pregnant women and pregnant people) to describe this population.

Who is it for?
• Healthcare practitioners
• Commissioners of health and social care services
• People with suspected or confirmed vitamin B12 deficiency, including deficiency caused by autoimmune gastritis

What does it include?
• the recommendations
• recommendations for research
• rationale and impact sections that explain why the committee made the recommendations and how they might affect practice
• the guideline context.

P.8 Common risk factors for vitamin B12 deficiency include recreational nitrous oxide use.

P.9 If a person has suspected vitamin B12 deficiency caused by recreational nitrous oxide use:

• use homocysteine as the initial test or
• if the test is not available in primary care, refer them to secondary care to have it.

P.14 Nitrous oxide-induced deficiency

1.5.8 Offer either intramuscular or oral vitamin B12 replacement to people with a vitamin B12 deficiency caused by nitrous oxide, based on the person’s preference. In July 2023, this was an off-label use of intramuscular vitamin B12 replacement. See NICE’s information on prescribing medicines.

1.5.9 If a person’s vitamin B12 deficiency is caused by recreational nitrous oxide use, advise them to stop using the substance.