Biomarkers

Can Vitamin B12 Deficiency Be a Sign of Cancer? Low and High B12 Explained

Low vitamin B12 is rarely a sign of cancer. Diet, age, acid-reducing medicines, metformin and absorption problems explain most low results. The real links run through pernicious anemia, stomach surgery and, in the other direction, unexplained high B12. Here is how the tests tell the causes apart.

Published

October 6, 2026

Read time

10 min read

Medically reviewed by

Rhonda Collins, FNP-C

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Low vitamin B12 is rarely a sign of cancer. Most low results come from low intake, reduced absorption with age, long-term acid-reducing medicines, metformin, or a gut condition such as celiac disease (NIH ODS). The genuine cancer connections are narrower. Pernicious anemia, an autoimmune cause of low B12, comes with a higher risk of stomach cancer (Vannella et al., 2013). Surgery that removes part of the stomach can cause low B12. And in the opposite direction, an unexplained high B12 has been associated with a cancer diagnosis in the following year in large cohort studies (Arendt et al., JNCI 2013).

The useful question is not “is this cancer?” but “why is my B12 low, or high?” A small set of blood tests answers that for most people: B12 with methylmalonic acid (MMA), a complete blood count with red cell size (MCV), ferritin, and, when the cause looks autoimmune, intrinsic factor and parietal cell antibodies.

The short answer

• Low B12 on its own is not a cancer warning. The common causes are diet, age-related changes in the stomach, medicines and absorption problems (NIH ODS).
• Pernicious anemia is the main low-B12 link to cancer. It raises the risk of stomach cancer and gastric carcinoid tumors (MedlinePlus), which is why finding the cause of a low result matters.
• Unexplained high B12 is the stronger statistical signal. In Danish and UK cohorts, people with high B12 who were not taking B12 had a higher chance of a cancer diagnosis within a year (Arendt et al., 2019). Supplements, liver disease and blood disorders are other reasons for a high level.
• Standard blood tests can usually find the cause. MMA confirms whether low B12 is affecting your cells, the blood count and ferritin show what kind of anemia is present, and antibody tests check for pernicious anemia.

How low B12 and cancer connect

There are four real connections between low B12 and cancer. In each, the cancer risk or the cancer treatment comes first and the low B12 follows. Low B12 is not known to cause cancer.

Pernicious anemia and atrophic gastritis

Pernicious anemia is an autoimmune disease in which the immune system attacks the stomach’s parietal cells, the cells that make intrinsic factor. Without intrinsic factor, the body cannot properly absorb B12 from food, and absorbs free B12 poorly as well (NIH ODS). The same autoimmune process thins the stomach lining, a condition called atrophic gastritis.

That damaged stomach lining is where the cancer link sits. The American Cancer Society and the National Cancer Institute both list pernicious anemia as a stomach cancer risk factor (ACS, NCI). In numbers:

• A 2013 systematic review of 27 studies estimated a nearly sevenfold relative risk of gastric cancer in pernicious anemia, with a pooled incidence of 0.27% per person-year, or about 2.7 cases per 1,000 people each year (Vannella et al.).
• An NCI analysis of US adults aged 66 and older found people with pernicious anemia had about twice the odds of noncardia gastric adenocarcinoma and about 11 times the odds of gastric carcinoid tumors (Murphy et al., 2015).

B12 treatment corrects the anemia but not the gastritis underneath it (Murphy et al.). That is the practical reason to identify pernicious anemia rather than simply topping up B12: the diagnosis changes the follow-up.

Stomach surgery, including surgery for cancer

Stomach surgery, whether to remove all or part of the stomach or for weight loss, can remove or bypass the cells that make stomach acid and intrinsic factor, so less B12 is absorbed (NIH ODS). People who have had a gastrectomy for stomach cancer are in this group. For them, low B12 is an expected effect of the operation and does not by itself suggest that the cancer has returned.

Previous stomach surgery for non-cancer reasons, such as ulcers, is also itself a stomach cancer risk factor (ACS).

Digestive cancers and absorption

B12 from food is released by stomach acid, binds intrinsic factor, and is absorbed at the far end of the small intestine, the ileum (NIH ODS). Disease or surgery at either end of that path can lower B12.

The colon is downstream of where B12 is absorbed. The anemia that can accompany colorectal cancer usually comes from slow bleeding into the bowel, and sometimes a low red blood cell count is the first sign (ACS). That pattern points to iron deficiency, not B12 deficiency, which is why ferritin belongs in the workup.

Cancer treatment effects

Some cancer treatments change B12-related results. Chemotherapy can enlarge red blood cells, the same high MCV that B12 or folate deficiency produces (MedlinePlus). And some chemotherapy, such as pemetrexed, is given with planned B12 injections and folic acid to reduce side effects (FDA label via DailyMed). During treatment, these results are read in the context of that treatment rather than on their own.

High B12 and cancer: what the association means

Unexplained high B12 has a more direct statistical link to cancer than low B12 does.

• In a Danish cohort of 333,667 people whose B12 was measured and who were not taking B12, cancer risk in the first year was about 3.4 times the population rate at 601 to 800 pmol/L and about 6.3 times at levels above 800 pmol/L. The excess was largest for blood cancers and for cancers related to smoking and alcohol (Arendt et al., JNCI 2013).
• A UK primary care cohort of 757,185 people found a similar pattern after adjusting for lifestyle factors. Those above 1,000 pmol/L had about 4.7 times the one-year cancer rate of people with normal levels, with the highest risks for liver and pancreatic cancers and myeloid blood cancers (Arendt et al., 2019).

What this does and does not mean:

• It is an association, concentrated in the first year. The researchers suggest that some existing cancers may affect B12 metabolism. The studies do not show that high B12 causes cancer (Arendt et al., 2019).
• These are relative risks. They compare groups. They do not mean that a high result usually comes with cancer.
• The studies excluded people taking B12. A high level while you take supplements or injections is a different finding, and usually an expected one.
• Liver disease and some blood disorders also raise B12. High vitamin B12: causes and when to follow up covers the full list and the usual next tests.

The far more common causes of low B12

These explain most low results, and several can apply at once (NIH ODS):

• Diet. Natural B12 comes only from animal foods, so vegans and many vegetarians have a higher risk unless they use fortified foods or supplements.
• Age. Depending on the definition, 3% to 43% of older adults living in the community have low B12, often linked with atrophic gastritis, which reduces the acid needed to release B12 from food.
• Acid-reducing medicines. In a Kaiser Permanente study of more than 25,000 people with B12 deficiency, two or more years of proton pump inhibitors (such as omeprazole) was associated with 1.65 times the odds of deficiency, and two or more years of H2 blockers (such as famotidine) with 1.25 times the odds (Lam et al., JAMA 2013).
• Metformin. The first-line diabetes and prediabetes medicine can reduce B12 absorption and lower serum levels.
• Celiac and Crohn’s disease. Rates of low B12 are higher in celiac disease. The evidence for Crohn’s disease is mixed.
• Pernicious anemia. It accounts for about 15% to 25% of B12 deficiency in older adults.
• Gut surgery, including gastric bypass and other weight-loss operations.

For what low B12 can feel like, see low vitamin B12 symptoms.

Warning signs that need prompt evaluation

Low B12 by itself is not an emergency. Some symptoms alongside it do deserve a prompt appointment, because they point to a cause that needs its own workup. The American Cancer Society lists these as possible signs of stomach cancer, while noting most are more likely caused by something else (ACS):

• weight loss without trying, or poor appetite;
• feeling full after a small meal, or ongoing indigestion or upper belly pain;
• vomiting, especially with blood;
• black or very dark stools;
• swelling in the abdomen.

Also get a prompt assessment for new numbness, weakness, trouble walking or confusion, because the nerve symptoms of B12 deficiency can occur without anemia (NIH ODS). And a high B12 when you are not taking B12 is worth following up rather than dismissing.

Which tests tell the causes apart, and why

Each test answers a different part of the question. Together they separate diet and medicine effects, pernicious anemia, iron loss and treatment effects in most people.

A few points explain the order:

• Start with B12, the blood count and ferritin. This shows whether B12 is low and whether iron loss is part of the picture. The Anemia Panel covers vitamin B12, folate, ferritin, iron studies and a complete blood count with MCV in one draw.
• Add MMA when B12 is borderline. The NIH notes MMA can confirm deficiency when serum B12 sits in the gray zone (NIH ODS). The MMA test is also available on its own.
• Check antibodies when there is no obvious cause. If diet, medicines and surgery do not explain the result, intrinsic factor and parietal cell antibodies check for pernicious anemia. Parietal cell antibodies appear in 85% to 90% of people with pernicious anemia but also in about 8% to 20% of healthy adults, so a positive result alone is not a diagnosis (Rusak et al., 2016).
• Ask about celiac testing if you also have digestive symptoms. How to test for celiac disease explains the antibody tests.

Test before you start supplements when you can, since B12 taken beforehand changes the result. For a price comparison across labs, see the vitamin B12 test cost guide.

What happens after a low result

A confirmed low B12 leads to two parallel steps: finding the cause, with the tests above, and replacing B12 in the form and route that suits that cause. The cause affects how B12 is replaced. Pernicious anemia, for example, blocks the normal absorption of B12 from food.

The B12 deficiency diagnosis guide covers the full workup, how results are staged and when to retest. Which B12 form to take compares the supplement forms, and how to improve your vitamin B12 naturally covers food sources.

If pernicious anemia is diagnosed, B12 treatment corrects the deficiency but not the atrophic gastritis behind it (Murphy et al.). Whether everyone with pernicious anemia needs routine stomach checks is not settled (Vannella et al.), so it is worth raising at diagnosis.

When to see a clinician

See a clinician soon if low B12 comes with any of the warning signs above, if it has no clear cause such as diet or a medicine, if it comes back after treatment, or if your B12 is high and you are not taking B12. Do not stop metformin or an acid-reducing medicine because of a B12 result. Ask the prescriber about monitoring or replacement instead.

Next step

If you have a low or unexplained B12 result, or symptoms that make you wonder, start with the Anemia Panel. It measures B12 and folate together with ferritin, iron studies and a complete blood count, so it shows both whether B12 is low and what kind of anemia, if any, sits alongside it. Add MMA or the antibody tests when the first results point there. Mito orders the tests and a clinician reviews your results. With Mito Membership, you also get a personalized action plan built on them.

Frequently asked questions

Does a normal B12 level rule out cancer?

No. Serum vitamin B12 measures B12 status and is not used to screen for cancer. A normal result is reassuring about B12, but new symptoms such as weight loss without trying or black stools still need their own evaluation.

Can taking B12 supplements cause cancer?

The evidence is mixed and mostly observational. The NIH Office of Dietary Supplements notes one cohort in which long-term supplemental B12 was linked with higher lung cancer risk in men but not women, and other studies that found no link. A high result while taking supplements is also a different finding from the unexplained high levels studied as a cancer marker.

Why do some cancer patients get B12 shots?

Some chemotherapy drugs need it. The FDA label for pemetrexed, used for some lung cancers, calls for vitamin B12 injections and daily folic acid before and during treatment to reduce blood and digestive side effects. That B12 protects against toxicity. It is not a cancer treatment.

Can colon cancer cause B12 deficiency?

Not usually. B12 is absorbed at the end of the small intestine, upstream of the colon. The anemia linked with colorectal cancer usually comes from slow bleeding, which causes iron deficiency, so ferritin and a blood count matter more than B12 for that question.