Vitamin B12 deficiency: who is at risk and how it is tested
What it found
Vitamin B12 deficiency affects about 2% to 3% of adults in the United States.
Testing is suggested for people who have at least one risk factor and one symptom.
What they found
Studies in people
How common it is
About 2% to 3% of adults in the United States have vitamin B12 deficiency.
Who is at risk
Risk factors include problems absorbing nutrients, eating little vitamin B12, taking certain medicines such as metformin and proton pump inhibitors, and being older.
Symptoms to look for
Symptoms depend on how severe the deficiency is. They may include fatigue, brain fog, depression, nerve damage in the limbs, and trouble with balance and coordination.
When to test
Screening everyone is not recommended. Testing should be considered for people who have at least one risk factor and one sign or symptom of deficiency.
Blood test results
A total serum vitamin B12 level below 180 pg/mL means deficiency. Levels between 180 and 350 pg/mL are borderline and need a methylmalonic acid measurement, which confirms deficiency if it is high.
Finding the cause
People without a clear cause should be tested for atrophic gastritis with a Helicobacter pylori test and for autoantibodies linked to autoimmune gastritis.
How it is treated
Most patients can take vitamin B12 by mouth, which works as well as an injection into the muscle. Injections should be considered for people with severe deficiency or nerve symptoms.
High B12 levels
Vitamin B12 levels that stay above 1,000 pg/mL on two measurements have been linked to solid tumors, blood cancers, and a higher risk of death from heart problems.
What the authors conclude
- They advise that testing should be considered in patients with at least one risk factor for and one clinical feature of vitamin B12 deficiency.
- They state that oral vitamin B12 supplementation can be used in most patients and is noninferior to intramuscular supplementation.
- They recommend that intramuscular administration should be considered in patients with severe deficiency or neurologic manifestations.
How it was done
The authors wrote this to answer common questions about who gets vitamin B12 deficiency, how it is found, and how it is treated.
What it can’t tell you
- We summarised it from the abstract only.
- This is a review of existing knowledge, not a new study, so it cannot prove what causes deficiency or what treatment works best for any one person.
- The links between high vitamin B12 levels and cancer or heart death come from associations, so they cannot show that high B12 causes these problems.
Who paid
- Authors work at
- The Ohio State University, Columbus
The paper
- Title
- Vitamin B12 Deficiency: Common Questions and Answers
- Type
- ReviewThe authors read earlier studies and describe what they found. It isn’t a new study.
- Evidence
- Studies in people
- Summarised from
- Abstract only
- Cite
- Patel H, McGuirk R (2025). Vitamin B12 Deficiency: Common Questions and Answers. American family physician.PubMed 40961307
Summary written 26 Sep 2026. Check it against the paper before it changes what you eat. How we summarise papers · Report an error