by Jack Norris, Registered Dietitian
For some time, there have been concerns that supplementing with either B12 or folic acid (the synthetic version of folate) may raise blood levels high enough to increase cancer risk. These concerns are somewhat related because I recommend multivitamins as an option for obtaining vitamin B12 for vegans, and most multivitamins contain folic acid, typically 400 µg.
Because vegans don’t get enough vitamin B12 from food, unless they regularly eat significant amounts of B12-fortified foods, we usually recommend that vegans take supplements in doses much higher than the RDA. The high doses make up for the infrequency with which they’re taken; often just a few times a week rather than with every meal.
Due to the concerns about cancer, I recently completed two literature reviews of the potential negative health impacts of taking vitamin B12 or folic acid. These reviews resulted in one change to our recommendations. Because the Institute of Medicine hasn’t set an upper limit for vitamin B12, and people would ask us how much is safe to take, we recommended that people not take more than 1,000 µg per day, which we explicitly stated was arbitrary. Upon completing these reviews, I now recommend that people not exceed the upper range of VeganHealth.org’s recommended doses.
Recent advances in AI have transformed how I’m able to research these topics. I used AI to search the literature and read papers far faster than I could alone, but the analysis, judgment calls, and writing reflect over 100 hours of my own work.
Below are the summaries. If you click through to the full article, you can see different levels of detail depending on whether you read only the introduction, the rest of the text, or the tables of the study details.
Folic Acid: Does It Make a Multivitamin Unsafe?
Having reviewed the evidence, I don’t think healthy adults need to avoid multivitamins containing folic acid at the typical dose of 400 µg/day.
Folic acid is the synthetic form of folate (vitamin B9) used in fortified foods and supplements. It can prevent neural tube defects, which are serious birth defects of the brain and spinal cord. Because the neural tube closes just 28 days after conception, often before someone knows they’re pregnant, adequate folic acid intake before and in the earliest weeks of pregnancy is critical. Women of childbearing age who don’t regularly eat fortified foods like bread, pasta, and rice are at risk of inadequate intake.
Some practitioners advise avoiding multivitamins that contain folic acid due to concerns about cancer. Almost all standard multivitamins contain folic acid, so this isn’t a trivial recommendation.
The European Food Safety Authority (2023), in a comprehensive review, set the upper intake level for folic acid at 1,000 µg per day. This limit was established not because cancer risk begins at that level, but to prevent folic acid from masking the early signs of vitamin B12 deficiency, a separate and well-characterized concern. The standard multivitamin dose of 400 µg is well below this threshold.
On cancer specifically, the evidence is mixed, voluminous, and difficult to summarize. Randomized trials and large prospective studies generally show no increased risk of cancer from folic acid intakes, including supplements at typical doses. Some studies find elevated risk with high blood folate levels, but these are difficult to interpret because people with undetected cancer may already have altered folate metabolism, making it look as though high folate caused the cancer when the reverse may be true. The EFSA concluded the evidence was insufficient to establish a causal relationship between folic acid intake and cancer risk at or below 1,000 µg per day.
For a healthy adult, a standard multivitamin containing 400 µg of folic acid appears safe based on the current evidence, and for women who could become pregnant, it may be important.
For the full review, see Folic Acid: Does It Make a Multivitamin Unsafe?
B12 Supplement Safety
Elevated vitamin B12 levels in the blood are usually a sign of an underlying health problem, not a cause of one. Liver disease, kidney disease, and various cancers can all raise B12 levels, which is why studies finding associations between high B12 and poor health outcomes are generally measuring the effect of these diseases on B12 levels, not vice versa.
For vegans who supplement with B12, the evidence is cautiously reassuring. The only large study of healthy adults examining B12 supplement safety found no association between B12 supplementation and increased mortality at any dose, including 1,000 µg/day or more.
There’s one area of genuine uncertainty: Some evidence suggests that very high circulating B12 levels may modestly increase lung cancer risk. The absolute risk is small; roughly 1 in 2,000 people with high B12 levels would develop one additional lung cancer case over six years, and the evidence isn’t conclusive. This isn’t a reason to stop supplementing, but rather to stick to recommended doses rather than taking more than needed. VeganHealth’s recommended supplemental doses result in roughly half of the total B12 absorbed compared to our estimated absorption for the high-risk group in that study.
People with kidney disease should avoid high-dose cyanocobalamin supplements specifically, as there is evidence it may accelerate kidney function decline. Methylcobalamin is a better option for this group.
Supplement with B12 at recommended doses, don’t take more than you need, and don’t be alarmed by studies linking high blood B12 to disease.
For the full review, see B12 Supplement Safety.
