Vitamin B12 deficiency, testing and treatment in adults
What it found
About 2% to 3% of US adults have low vitamin B12, which can cause tiredness, brain fog and nerve problems.
Oral supplements work as well as injections for most people.
What they found
Studies in people
How common it is
Low vitamin B12 affects roughly 2% to 3% of adults in the United States. Older age raises the chance of having it.
Risk factors
Trouble absorbing nutrients, eating little vitamin B12 and older age raise the risk. Certain medicines like metformin and stomach acid blockers also play a role.
Symptoms
Signs depend on how low levels are. They can include tiredness, brain fog, depression, nerve damage and trouble with balance.
Testing levels
A blood level below 180 pg/mL points to a shortage. Borderline results between 180 and 350 pg/mL need a second test called methylmalonic acid to confirm it.
Pills versus injections
Taking vitamin B12 by mouth works just as well as getting shots for most patients. Shots are suggested when the shortage is severe or causes nerve problems.
Very high levels
Blood readings above 1,000 pg/mL on two tests have been linked to certain cancers and a higher chance of dying from heart disease.
What the authors conclude
- They advise doctors to test for low vitamin B12 when a patient has at least one risk factor and one symptom.
- They say oral supplements work as well as injections for most patients, but shots should be used for severe cases or nerve problems.
- They note that very high vitamin B12 readings may signal other serious health issues.
How it was done
The authors wanted to answer common questions about spotting and treating low vitamin B12 levels in patients.
They reviewed existing research on how often vitamin B12 deficiency happens, what causes it, how to test for it and how to treat it. The work focused on adult patients in the United States.
What it can’t tell you
- We summarised it from the abstract only.
- This work gathers past research rather than running a new trial, so it cannot prove that treatments directly cause improvements.
- Links between very high vitamin B12 levels and cancer or heart death do not show that high levels cause those conditions.
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 30 Sep 2026. Check it against the paper before it changes what you eat. How we summarise papers · Report an error