Vitamin B12 Cancer Risk: Scientists Warn That Too Much or Too Little Could Be Harmful

The Vitamin B12 cancer risk debate has taken a new turn after researchers shared fresh findings suggesting that both low and excessively high levels of this essential nutrient may carry health concerns. While B12 is crucial for many of the body’s most important functions, scientists are now urging caution about how much people consume — especially through supplements.

Why Vitamin B12 Matters So Much

Vitamin B12 plays a foundational role in keeping the body running smoothly. It supports the production of red blood cells, helps maintain a healthy nervous system, and is essential to how cells multiply and repair DNA.

Since the body cannot produce B12 on its own, people must get it from food. When intake is too low, the deficiency can quietly cause a wide range of symptoms — and in some cases, lead to lasting nerve or cognitive damage if left untreated.

Where You Can Get B12 Naturally

B12 is naturally found in animal-based foods, including:

Meat and poultry

Fish and seafood

Eggs

Milk and cheese

For people who don’t eat animal products, certain cereals and breads are fortified with B12 to help fill the gap. Most individuals consuming a balanced diet get enough of this nutrient. However, vegans, older adults whose bodies absorb nutrients less efficiently, and people with specific digestive disorders may need supplements to maintain healthy levels.

What the New Research Reveals

Writing for The Conversation, Ahmed Elbediwy, Senior Lecturer in Cancer Biology and Clinical Biochemistry at Kingston University, and Nadine Wehida, Senior Lecturer in Genetics and Molecular Biology at the same institution, explored the growing scientific interest in B12’s relationship with cancer.

According to the experts, scientists have been asking whether unusually high B12 intake — or elevated B12 levels in the bloodstream — could be linked to cancer development. Their explanation highlights why both extremes can be problematic.

When B12 levels drop too low, DNA replication errors become more likely. Over time, these mistakes can lead to mutations that may increase the risk of certain cancers, particularly colon cancer. That’s one reason deficiency is taken so seriously by doctors.

The U-Shaped Connection Between B12 and Cancer

A 2025 case-control study conducted in Vietnam discovered what researchers describe as a U-shaped relationship between B12 intake and cancer risk. In simple terms, both very low and very high levels appeared to be tied to higher risk.

However, the researchers emphasised that this type of study can only point to associations, not prove cause and effect. The takeaway, they explained, is not that B12 is dangerous — rather, balance is essential.

Can Extra B12 Protect Against Cancer?

It might seem reasonable to assume that since B12 supports healthy cell function, more of it would offer added protection. But the science doesn’t back up that idea.

B12 fuels the growth of all cells, not just healthy ones. Experts caution that if pre-cancerous cells are already developing in the body, an oversupply of growth-supporting nutrients like B12 could, in theory, encourage them to grow as well. While this remains difficult to confirm in humans, it raises valid concerns.

Long-term studies of high-dose B vitamin supplements have not shown convincing evidence that they reduce cancer cases or cancer-related deaths.

Possible Links to Lung Cancer

Some observational studies have hinted at a small rise in lung cancer risk among people taking high doses of B6 and B12 over many years — especially in men and smokers. Researchers caution that these studies cannot confirm whether the supplements directly caused the cancers, but the pattern is enough to warrant attention.

Is High B12 a Cause or a Clue?

The conversation around B12 and cancer became more prominent after doctors noticed unusually elevated levels in some cancer patients. A 2022 study clarified that B12 often appears in the blood alongside cancer but does not appear to trigger the disease itself.

In fact, researchers are increasingly looking at high B12 as a possible warning marker rather than a cause. A large 2026 study revealed that colon cancer patients with extremely high B12 levels had a median survival of around five years, compared with nearly eleven years for those whose B12 levels were normal.

Similar trends have been observed in oral cancer and in patients receiving immunotherapy, where high B12 levels have been linked to poorer outcomes. Persistent and unexplained high B12 — particularly when not caused by supplements — may signal underlying issues such as liver problems, blood disorders, or even undetected cancer.

Should You Be Worried About Your Diet?

For most people, everyday eating is not a cause for concern. It’s extremely hard to consume excessive amounts of B12 from food alone. Deficiency, in fact, is far more common than overconsumption.

The real worry, experts say, lies with prolonged high-dose supplementation taken without medical guidance, or with blood tests showing consistently high B12 levels in someone who isn’t taking supplements.

The Bottom Line: More Isn’t Always Better

The researchers stressed that when it comes to B12, “more is not better.” Their key advice includes:

Get enough B12 through food or supplements if you genuinely need them.

Pay extra attention if you’re vegan, older, or have a condition that affects nutrient absorption.

Avoid mega-doses unless a doctor specifically recommends them.

Maintain a balanced diet and healthy lifestyle overall.

As with most nutrients, the goal isn’t to consume the highest amount possible — it’s to get the right amount for your body. When it comes to vitamin B12, that balance could make all the difference for long-term health.