Vitamin B12 has not been proven to cause cancer, but the relationship between this essential nutrient and cancer risk is more complicated than a simple yes or no. Several large studies have found that people with high blood levels of B12 are diagnosed with cancer more often, and at least one genetic analysis suggests that higher B12 may genuinely raise colorectal cancer risk by about 20%. Yet the largest pooled analysis of supplement trials found no significant increase in cancer incidence or death among people taking B vitamins. The picture depends heavily on which cancer, which population, and whether the B12 was measured in the blood or taken as a supplement.
High Blood B12 Is Often a Consequence of Cancer, Not a Cause
One of the most common reasons people worry about B12 and cancer is a blood test showing unusually high levels. A large Danish population-based study found that people with elevated plasma B12 were more likely to be diagnosed with various cancers, particularly blood cancers, within the first year after the measurement.1JNCI: Journal of the National Cancer Institute. Elevated Plasma Vitamin B12 Levels as a Marker for Cancer: A Population-Based Cohort Study That timing is the key clue: if B12 were causing the cancer, you would expect the diagnosis to come years later, not within months. What is more likely is that the cancer itself, or the body’s response to it, pushes B12 levels up.
Cancers can raise blood B12 through several routes. Tumors that affect the liver can release stored B12 into the bloodstream. Blood cancers cause rapid white blood cell turnover, and those cells produce transport proteins called haptocorrins that carry B12.2PubMed Central. Hypervitaminemia B12 in the Elderly: A Forgotten Marker of Serious Underlying Diseases Even kidney failure and chronic inflammation can paradoxically raise B12 levels. A French hospital study found that patients whose plasma B12 stayed persistently above 1,000 ng/mL had a dramatically higher risk of being diagnosed with a solid tumor, with about a six-fold increase compared to patients whose levels stayed normal.3PubMed Central. Persistent elevation of plasma vitamin B12 is strongly associated with solid cancer But even the authors framed this as a diagnostic red flag, not as evidence that B12 was fueling the disease. In clinical terms, unexplained high B12 on a blood test is sometimes treated as a reason to look for hidden cancer, similar to how an elevated sedimentation rate prompts a search for inflammation.
The Lung Cancer Signal in Men Taking High-Dose Supplements
The single most alarming finding about B12 and cancer comes from a large U.S. prospective study called the Vitamins and Lifestyle (VITAL) cohort. Researchers followed over 77,000 adults for about a decade and found that men who took high-dose B12 supplements on their own, not as part of a multivitamin, had roughly double the risk of lung cancer compared to men who did not supplement. The highest-risk group was men taking more than 55 micrograms per day for ten years, with a hazard ratio of about 2.0. Among men who were also current smokers, the risk was even higher.4PubMed Central. Long-Term, Supplemental, One-Carbon Metabolism-Related Vitamin B Use in Relation to Lung Cancer Risk in the Vitamins and Lifestyle (VITAL) Cohort Vitamin B6 showed a similar pattern in the same study, and the increased risk applied only to men, not women.
A separate prospective cohort study of dietary B12 intake, rather than supplements, also found a positive link: an 18% increased risk of lung cancer in the highest-intake group after accounting for smoking and other variables.5PubMed Central. The Association Between Dietary Vitamin B12 and Lung Cancer Risk: Findings from a Prospective Cohort Study And a pooled analysis of multiple cohorts in the LC3 consortium, which included direct measurements of circulating B12, found a 15% increased lung cancer risk for each doubling of blood B12 levels. A Mendelian randomization analysis using genetic variants that influence B12 levels supported the association, with genetically predicted higher B12 tied to about an 8% increase in lung cancer risk.6PubMed Central. Is high vitamin B12 status a cause of lung cancer?
These three lines of evidence, observational supplements data, dietary intake data, and genetic prediction, all pointing in the same direction give the lung cancer association more weight than any other cancer type. Still, the VITAL finding has drawn some criticism because men who self-select into high-dose supplement use for years may differ from non-users in ways that are hard to measure, and smoking remains the overwhelmingly dominant risk factor for lung cancer.
Colorectal Cancer and the Genetic Evidence
Mendelian randomization studies, which use inherited genetic variants as proxies for lifelong exposure, have provided some of the most compelling evidence of a possible causal link between B12 and colorectal cancer specifically. A 2023 analysis covering both the UK Biobank and Finnish health registry data found that genetically predicted higher B12 was associated with about a 22% increase in colorectal cancer risk, a result that passed strict statistical correction for testing many nutrients and many cancers at once.7PubMed Central. An atlas of associations between 14 micronutrients and 22 cancer outcomes: Mendelian randomization analyses A separate Mendelian randomization study also found a 16% increased colorectal cancer risk per standard-deviation increase in genetically predicted B12.8British Journal of Cancer. Genetically predicted circulating B vitamins in relation to digestive system cancers
Genetic analyses like these are less vulnerable to the reverse-causation problem because your gene variants are set at conception, long before any cancer develops. But they are not perfect. The genetic instruments used to proxy B12 levels may also affect other metabolic pathways, making it hard to isolate B12 itself as the culprit.
Interestingly, a large multi-institutional retrospective study found that people with low serum B12 also had a higher risk of colorectal cancer, with a 16% increase compared to matched controls. That association weakened when the researchers imposed longer lag times between the blood draw and diagnosis, suggesting some of the signal may again reflect early disease rather than true causation. But the fact that both high and low B12 have shown associations with colorectal cancer in different study designs hints at a U-shaped relationship, or at the very least, a complicated one where simple “more is bad” framing does not fully capture reality.
Prostate Cancer
Prostate cancer has attracted attention because some observational studies have found strikingly large associations. A Norwegian prospective study found that men in the highest quartile of plasma B12 had nearly three times the prostate cancer risk of men in the lowest quartile, and the association held up after adjusting for folate, body weight, and smoking.9PubMed. Plasma folate, vitamin B12, and homocysteine and prostate cancer risk: a prospective study A larger pooled analysis of six cohorts tempered that number considerably, finding only a 12% increase in risk for the top versus bottom fifth of circulating B12.10PubMed Central. Circulating Folate and Vitamin B12 and Risk of Prostate Cancer: A Collaborative Analysis of Individual Participant Data from Six Cohorts Including 6875 Cases and 8104 Controls
A Mendelian randomization study tried to settle whether the link is causal but found that confidence intervals were too wide to draw any firm conclusions, largely because too few genetic variants explained too little of the variation in B12 levels.11PubMed Central. The causal roles of vitamin B(12) and transcobalamin in prostate cancer: can Mendelian randomization analysis provide definitive answers? In practical terms, the prostate cancer data remains suggestive but inconclusive.
Supplement Trials Show No Clear Increase Overall
The most reassuring evidence comes from clinical trials where researchers actually gave people B vitamin supplements and tracked cancer outcomes over years. A meta-analysis pooling data from over 73,000 participants found that B vitamin supplementation produced a non-significant 4% increase in cancer incidence and a non-significant 5% increase in cancer deaths.12PubMed Central. Effect of vitamin B supplementation on cancer incidence, death due to cancer, and total mortality Those point estimates lean slightly in the wrong direction, but statistically the results were indistinguishable from no effect at all. Total mortality was completely unchanged.
This is the kind of evidence that should carry the most weight for people wondering whether their daily B12 supplement is dangerous. Observational studies can detect associations, and genetic analyses can suggest causal directions, but randomized trials are the gold standard for answering “does taking this actually increase cancer risk?” And across tens of thousands of participants, the answer so far has been no, at least at the doses used in the trials that were included. A comprehensive scoping review of the human evidence reached a similar conclusion: there is not enough evidence to assume that high plasma B12, high intake, or pharmacological doses of B12 are causally related to cancer.13PubMed Central. High Plasma Vitamin B12 and Cancer in Human Studies: A Scoping Review to Judge Causality and Alternative Explanations
The tension between the VITAL cohort’s findings about high-dose supplements and the broader meta-analysis of trials is worth acknowledging. The VITAL study was observational, meaning it tracked people’s self-reported supplement habits rather than randomly assigning them to high doses. And the trials pooled in the meta-analysis typically used moderate doses for cardiovascular or neurological endpoints, not the mega-doses some people take on their own. So the question of whether very high, long-term supplementation in specific subgroups (particularly male smokers) carries meaningful lung cancer risk has not been fully addressed by trials.
Why B12 Could Plausibly Affect Cancer Biology
B12 is a cofactor for enzymes in what biochemists call one-carbon metabolism, a network of reactions the body uses to build DNA, produce amino acids, and add chemical tags (methyl groups) to DNA that switch genes on or off.14PubMed Central. B Vitamins and One-Carbon Metabolism: Implications in Human Health and Disease Because cancer is fundamentally a disease of uncontrolled cell division, and cell division requires a lot of DNA synthesis, nutrients that fuel DNA production are at least theoretically capable of accelerating tumor growth once a cancer has already started.
Research in human gut cells has shown that B12 availability influences how methyl tags are distributed across the genome, affecting genes related to cell growth and intestinal barrier function.15PubMed Central. Vitamin B12 Regulates the Transcriptional, Metabolic, and Epigenetic Programing in Human Ileal Epithelial Cells In colorectal cancer patients, higher circulating B12 was associated with decreased methylation of certain repetitive DNA sequences in tumor tissue, a pattern linked to genomic instability.16PubMed Central. Association between Serum Vitamin B12 and Global DNA Methylation in Colorectal Cancer Patients These findings sketch out a plausible biological pathway, but they fall short of proving that dietary or supplemental B12 in healthy people can initiate cancer. They are more consistent with a scenario where extra B12 feeds a tumor that already exists.
The Folate Complication
B12 and folate (vitamin B9) work together in the same metabolic pathways, and teasing them apart in research is difficult. A large Norwegian randomized trial that gave patients folic acid combined with B12 found increased cancer incidence and mortality in the treatment group, but the statistical analysis pointed to folate as the primary driver of the adverse effect, not B12.17JAMA. Cancer Incidence and Mortality After Treatment With Folic Acid and Vitamin B12 In a European breast cancer study, B12 was positively associated with breast cancer risk, but only in women whose folate levels were below the median, suggesting that the balance between the two nutrients matters more than the absolute level of either.18PubMed. Biomarkers of folate and vitamin B12 and breast cancer risk: report from the EPIC cohort
This interplay means that many studies attributing risk to B12 may be partially capturing the effect of folate, or of an imbalance between the two. Countries that fortify their grain supply with folic acid, including the United States and Canada, create a baseline folate status in the population that could modify whatever effect B12 has. Researchers have not yet untangled this fully.
Cancer Cells Appear to Crave B12
A growing body of lab research has found that many cancers overexpress CD320, a receptor on the cell surface that pulls in the form of B12 bound to its transport protein. CD320 has been confirmed as overexpressed in several cancer types, and researchers are investigating it as both a biomarker for early detection and a doorway for delivering cytotoxic drugs directly to tumors.19PubMed Central. CD320 Receptor and Vitamin B12 as Potential Targets for Anti-Cancer Therapy In triple-negative breast cancer models, cancer stem cells showed significantly increased CD320 expression compared to non-stem cancer cells.20PubMed Central. CD320 expression is increased in breast cancer stem cells but does not promote their expansion or alter key histone methylation marks
This does not mean eating B12-rich foods feeds your tumor. Even without supplements, circulating B12 is available to all cells. What the CD320 research suggests is that cancer cells have evolved to be especially good at grabbing B12 from the bloodstream, presumably because their rapid division demands more of the raw materials B12 helps provide. The therapeutic angle here is actually the opposite of the “B12 causes cancer” fear: scientists are trying to exploit cancer’s hunger for B12 by attaching drugs and nanoparticles to B12 molecules, turning the nutrient into a Trojan horse.
B12 in Active Cancer Treatment
If B12 truly promoted cancer growth, you would expect oncologists to avoid giving it to patients. In practice, the opposite happens. Pemetrexed, a widely used chemotherapy drug for non-small-cell lung cancer and mesothelioma, works by disrupting the same folate-dependent pathways that B12 participates in. To reduce the drug’s severe side effects, patients are routinely given B12 injections along with folic acid before and during treatment. A randomized trial confirmed that starting B12 supplementation at the same time as pemetrexed is safe and does not increase toxicity.21PubMed. Timing of folic acid/vitamin B12 supplementation and hematologic toxicity during first-line treatment of patients with nonsquamous non-small cell lung cancer using pemetrexed-based chemotherapy: The PEMVITASTART randomized trial In a phase II mesothelioma trial, patients who received B12 and folate supplementation survived a median of 13 months versus 8 months for those who did not, and they tolerated more cycles of chemotherapy.22PubMed. Phase II study of pemetrexed with and without folic acid and vitamin B12 as front-line therapy in malignant pleural mesothelioma
Gastric cancer patients who have had their stomachs removed lose the ability to absorb B12 from food, and they are treated with replacement B12 without concern about promoting recurrence. A study of post-gastrectomy patients found that oral B12 effectively corrected deficiency and relieved neurological symptoms with no adverse effects.23PubMed. Oral vitamin B12 replacement: an effective treatment for vitamin B12 deficiency after total gastrectomy in gastric cancer patients
Gut Bacteria, B12, and Colorectal Tumors
An intriguing wrinkle in the colorectal cancer story involves the gut microbiome. A study that tested 157 gut bacterial species against colorectal cancer cell lines found that bacteria from the Bacteroidaceae family that promoted cancer cell growth were enriched for genes involved in cobalamin (B12) synthesis.24PubMed Central. Growth rate alterations of human colorectal cancer cells by 157 gut bacteria This raises the possibility that some of the observed link between B12 and colorectal cancer is mediated not by dietary B12 intake but by the metabolic activity of gut bacteria that both produce B12 locally and independently promote tumor growth. It is one more factor that makes isolating B12 as the causal agent extremely difficult.
What This Means if You Take B12 Supplements
If you are taking a standard B12 supplement because of a diagnosed deficiency, a vegan or vegetarian diet, or age-related absorption issues, the current evidence does not support stopping. Deficiency itself causes real harm: nerve damage, anemia, cognitive problems. The doses found in most daily supplements and multivitamins (typically 2.4 to 100 micrograms) are far below the mega-doses flagged in the VITAL study.
Where caution may be warranted is in men, particularly smokers or former smokers, who take very high individual B12 supplements over many years for no documented medical reason. The VITAL finding of roughly doubled lung cancer risk in men taking more than 55 micrograms per day for a decade is specific enough that it deserves attention, even though it came from an observational study. There is no established upper limit for B12 toxicity in the way there is for fat-soluble vitamins, precisely because excess B12 has historically been considered harmless. That assumption may need revisiting for long-term high-dose use in certain populations.
For anyone with an unexplained high B12 reading on a blood test who has not been taking supplements, the more pressing concern is what is causing the elevation. Liver disease, kidney problems, and occult malignancy should all be on the differential. The B12 level itself is not the threat; it is a flag that something else may be going on.