Could High B12 Levels Be a Sign of Cancer? Here’s What You Should Know

ByBob Burns

Updated April 02, 2025

So What’s the Deal with High B12?

Okay, let’s dig a little deeper. When we say “high B12,” we’re usually talking about serum (blood) B12 levels that are well above the reference range, which is often something like 200–900 pg/mL. Some folks see numbers over 1000 and freak out—understandably!

But it’s not always cause for alarm.

A Vitamin You Can’t Overdose On?

Interestingly, B12 is water-soluble, which means your body pees out the excess—most of the time. That’s why toxicity from B12 supplements is rare. If you’re taking supplements or getting regular B12 injections, your high numbers probably make total sense.

But if you’re not supplementing and still seeing unusually high levels? That’s when doctors may want to rule a few things out.

Can High B12 Levels Point to Cancer?

The short answer: Sometimes. But let’s be clear—this is not the most common reason for elevated B12.

According to research published in the journal Clinical Nutrition, persistent high B12 levels—especially in people not supplementing—have been associated with certain cancers, including:

• Blood cancers like leukemia or lymphoma
• Liver cancer
• Some types of gastrointestinal cancers

Here’s the thing, though. These associations are rare and usually found in people who already had other symptoms or risk factors. Elevated B12 may be due to your liver or blood cells releasing stored B12 into your bloodstream—not because B12 is causing harm, but because something else is going on in your body.

So, high B12 might be more of a “witness” to a problem, not the “criminal,” if that makes sense.

Sources to explore:

• Mayo Clinic: Vitamin B12 Test
• Healthline: Vitamin B12 Levels
• WebMD: What High B12 Can Mean

Other Possible Causes of High B12

Before jumping to the worst-case scenario, know this: high B12 can happen for a bunch of other (often less scary) reasons:

• Liver conditions like hepatitis or fatty liver disease
• Kidney dysfunction, which affects how B12 is cleared from the body
• Blood disorders like polycythemia vera
• Recent B12 injections or supplements
• Gut issues that affect absorption and trigger release of stored B12

In fact, studies suggest that around 10–20% of people with high B12 have no identifiable cause—and no related health concerns either.

What Should You Do If Your B12 Is High?

Step one? Don’t panic.

Step two? Look at the full picture—are you experiencing other symptoms like fatigue, unexplained weight loss, digestive issues, or changes in appetite? Are you on any medications or have a condition that might affect your liver or kidneys?

Step three? Talk to your doctor. They’ll likely review your medical history, ask about supplements, and possibly run some additional tests.

Sometimes, they’ll check something called “active B12” or holotranscobalamin, which gives a better idea of how much usable B12 your body has. Serum B12 alone doesn’t always tell the full story.

The Takeaway

Yes, high B12 can sometimes be linked to cancer—but that’s not the most likely explanation. In many cases, it’s totally benign or tied to something else entirely.

So if you’ve just spotted a high number on your lab report, don’t assume the worst. Think of it as a nudge to check in with your body—and your doctor.

Your B12 story depends on you—your health history, your diet, your age, your medications, and more. A single lab result never tells the whole tale.