If a blood test shows low B12, it is natural to wonder what caused it and whether it could mean something serious like cancer. The honest answer: usually the explanation is diet, absorption or a medication. But B12 levels can occasionally be a clue worth following up, which is exactly why a doctor should interpret them.
Written by Vita Globe Editorial Team | Reviewed against NIH guidance, clinical reviews and peer-reviewed research
Can vitamin B12 deficiency be a sign of cancer?
Low B12 is not a sign of cancer by itself. It is most often caused by low intake, age-related absorption problems, the autoimmune condition pernicious anemia, digestive conditions or surgery, or medications like metformin[1][2]. Pernicious anemia is linked to a higher risk of stomach cancer, which is why diagnosed patients may be monitored[3]. Separately, unexplained high B12 has been associated with cancer in some studies[4].
Caveat: Only your doctor can work out why your levels are off. Please don't use this article, or a supplement, to rule anything in or out.
Sources: NIH ODS · Vannella et al., 2013 · Arendt et al., JNCI 2013 · Verified 2026-09-27
IN THIS ARTICLE
What usually causes low B12?
Most cases of low B12 have ordinary explanations. The main causes are[1][2][5]:
- Reduced absorption with age, often linked to lower stomach acid (atrophic gastritis).
- Diets low in animal foods, such as vegan diets without fortified foods or supplements.
- Pernicious anemia, an autoimmune condition that blocks intrinsic factor, the protein needed to absorb B12.
- Digestive conditions and surgery, such as celiac disease, Crohn's disease, or weight-loss or stomach surgery.
- Medications, especially long-term metformin and acid-reducing drugs (PPIs and H2 blockers).
For a full list of symptoms and risk factors, see Vitamin B12 Deficiency: Signs, Causes, and Who Is Actually at Risk.
When can low B12 point to something that needs attention?
Sometimes low B12 is the first clue to an underlying condition that deserves a proper diagnosis. Pernicious anemia is the main example. It damages the stomach lining over time, and a systematic review found that people with pernicious anemia have a higher rate of stomach (gastric) cancer than the general population[3]. That is why doctors may recommend follow-up for some people with this diagnosis.
Low B12 can also reflect digestive diseases that affect absorption, which are worth identifying for their own sake[6]. And deficiency itself can cause nerve damage if it goes untreated for a long time, so it deserves attention even when the cause is simple[2].
The key message: low B12 is a reason to find out why, not a reason to panic.
What about high B12 levels and cancer?
You may have seen headlines linking B12 and cancer. Most of this research is actually about unexpectedly high blood B12, not low. In a large Danish population study, people with high plasma B12 levels were more likely to be diagnosed with cancer, mostly within the first year after the test[4]. One likely explanation is that the high levels reflect changes caused by an illness that is already present, rather than B12 causing cancer.
Two important caveats. First, this is an association in blood tests, not proof that B12 intake causes cancer. Second, taking B12 supplements will raise your blood levels, which is expected. Always tell your doctor about any supplements you take so they can interpret results correctly[1].
When should you see a doctor about B12?
Talk to your healthcare provider if you have symptoms of deficiency, such as ongoing fatigue, numbness or tingling, problems with balance, a sore or smooth tongue, or memory or mood changes, or if a routine test shows low or unusually high B12[5][7]. They may check a full blood count, repeat the B12 test, or order extra tests (such as intrinsic factor antibodies) to find the cause[7].
Seek care promptly for new neurological symptoms, unexplained weight loss, or changes in digestion. These need a proper evaluation whatever your B12 level shows.
The honest summary
- Low B12 usually has an everyday cause like diet, age, or medication.
- Sometimes it signals a condition such as pernicious anemia, which carries a higher risk of stomach cancer and needs a doctor's follow-up.
- Research linking B12 to cancer mostly involves unexplained high levels, and it shows association, not cause.
- Don't self-treat symptoms that worry you. Get tested first so the cause isn't hidden.
Our option
Vita Globe Vitamin B12 Gummies for Adults
If your healthcare provider has ruled out an underlying cause and suggests keeping B12 in your routine, our B12 Gummies for Adults provide 1,000mcg of vitamin B12 (as cyanocobalamin) per two-gummy serving. B12 supports red blood cell formation and cellular energy production.* A supplement is not a substitute for testing or medical care.
Frequently asked questions
What cancers are linked to B12 deficiency?
The best-documented link is between pernicious anemia, an autoimmune cause of B12 deficiency, and a higher risk of stomach cancer[3]. Low B12 in general is not considered a cancer marker.
What diseases cause low B12?
Pernicious anemia, atrophic gastritis, celiac disease, Crohn's disease and stomach or bowel surgery can all reduce B12 absorption[1][5].
Should I worry if my B12 is high?
High B12 is often due to supplements or fortified foods. If it is high and you don't take supplements, your doctor may want to look into why, based on studies linking unexplained high levels with other conditions[4].
Should I take B12 before my blood test?
Ask your doctor. Taking B12 before testing can raise your levels and make results harder to interpret, so mention any supplements you use.
Sources
- Office of Dietary Supplements, National Institutes of Health. Vitamin B12: Fact Sheet for Health Professionals. Updated July 2, 2025. ods.od.nih.gov
- Stabler SP. Vitamin B12 deficiency. N Engl J Med. 2013;368(2):149-160. doi:10.1056/NEJMcp1113996
- Vannella L, Lahner E, Osborn J, Annibale B. Systematic review: gastric cancer incidence in pernicious anaemia. Aliment Pharmacol Ther. 2013;37(4):375-382. doi:10.1111/apt.12177
- Arendt JFB, Pedersen L, Nexo E, Sørensen HT. Elevated plasma vitamin B12 levels as a marker for cancer: a population-based cohort study. J Natl Cancer Inst. 2013;105(23):1799-1805. doi:10.1093/jnci/djt315
- Langan RC, Goodbred AJ. Vitamin B12 deficiency: recognition and management. Am Fam Physician. 2017;96(6):384-389. aafp.org
- Green R, Allen LH, Bjørke-Monsen AL, et al. Vitamin B12 deficiency. Nat Rev Dis Primers. 2017;3:17040. doi:10.1038/nrdp.2017.40
- National Institute for Health and Care Excellence. Vitamin B12 deficiency in over 16s: diagnosis and management (NICE guideline NG239). 2024. nice.org.uk/guidance/ng239
About the Vita Globe Editorial Team
The Vita Globe Editorial Team researches and writes our wellness content, working from peer-reviewed research, systematic reviews and guidance from bodies such as the NIH and the FDA. We cite our sources inline and update articles when the evidence changes.
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. This article is for informational purposes only and is not a substitute for medical advice. Talk to your healthcare provider before starting any new supplement, particularly if you are pregnant, nursing, taking medication, or caring for a child.
