Updated on November 30, 2023.
Medically reviewed on January 23, 2023 by Morgan Spicer, Medical Communications Manager. To give you technically accurate, evidence-based information, content published on the Everlywell blog is reviewed by credentialed professionals with expertise in medical and bioscience fields.
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Whether you’re choosing supplements to help meet nutritional recommendations, or just stay apprised of the latest headlines in healthcare, few of us were excited to hear that a nutrient as essential for our bodies as vitamin B12 could potentially cause health problems.
While several studies have demonstrated a correlation between cancer and excessive levels of B12, the NIH emphasizes that evidence here remains inconclusive. That said, with a handful of studies to indicate the contrary, is the link valid or simply mere hearsay?
Below, we unpack the scenarios in which vitamin B12 is helpful versus harmful so you can make an informed decision about the supplements you take and your long-term health.
Vitamin B12, also known as cobalamin, is part of the vitamin B complex. This is a collection of eight vitamins that assist with everything from optimal cognitive function to converting the carbohydrates you consume into usable energy .
Stored in your liver, vitamin B12 is responsible for supporting a range of vital functions on its own. These include :
Interest in vitamin B12’s role has grown in recent years, and not just because it keeps our neurons firing, helps us learn new skills, and enhances our memories . In excessive amounts, some evidence shows vitamin B12 may be linked to heightened vulnerability to certain cancers.
esearch on the relationship between B12 and cancer is still in its early stages, and science-backed findings remain inconclusive. That said, there are some hypotheses to be aware of if you're concerned about the link between a high vitamin B12 level and cancer.
A large-scale 2017 study published in the Journal of Clinical Oncology showed that males who supplemented with “megadoses” of vitamin B6 and vitamin B12 had a 30 to 40% increased risk of lung cancer when compared to males and females who didn’t supplement .
However, this higher risk was discovered in male current smokers. Further, the male subjects took 23 times the recommended daily amount over the course of 10 years. Females who participated in the study did not demonstrate an increased risk of lung cancer.
Nonetheless, this led the study authors to conclude that supplementing with vitamin B12 should not be deemed a “chemopreventive” measure for lung cancer. In fact, doing so may have the opposite effect .
In a follow-up study, researchers found that supplementing with vitamins B12 and B9 (folic acid) may increase the risk of colon cancer. These two nutrients are frequently used to support bone health and reduce the risk of osteoporosis. However, researchers still maintain that a larger, more comprehensive study must be performed to render findings conclusive.
A 2013 study published in the Journal of the National Cancer Institute concluded that elevated serum cobalamin (B12) levels were associated with :
Ultimately, this study found that elevated B12 levels may be an important biomarker for cancer. The hypothesis was corroborated in a separate 2020 study, which showed that an unexplained high level of serum vitamin B12 might be linked to the development of “solid” cancers (those with tumors, as in breast cancer) . However, this theory requires further research .
It’s critical to note, however, that it’s unclear if the disease process itself causes elevated vitamin B12 levels . Further, the authors of the latter study stated that the increase in vitamin B12 was likely not related to standard vitamin B12 intake since foods and dietary supplements do not have a substantial effect on the amount of B12 in blood plasma.
On the other hand, an older study featured in the American Journal of Epidemiology reported no correlation between vitamin B12 and a heightened risk of lung cancer .
So, where does this leave us?
While evidence on the connection between vitamin B12 and cancer is varied, this makes it all the more important for individuals to evaluate their vitamin levels before taking a vitamin B12 supplement. Always consult with your healthcare practitioner before introducing any new vitamin or mineral into your diet.
As these findings demonstrate, the increased risk for lung cancer was due to incredibly high doses of vitamin B12—23 times more than the recommended daily amount of 2.4 micrograms .
Other research hypothesized that vitamin B12 levels became elevated during the disease process. This is because, like its cousins in the B-complex, vitamin B12 is a water-soluble vitamin, which means any B12 you don’t need is excreted through urine.
In the absence of vitamin B12 supplementation, high levels of vitamin B12 in your bloodstream may be indicative of more serious underlying health issues. These include :
Not necessarily, but it's recommended you consult with your healthcare provider if you suspect you have an elevated B12 level. This is particularly important if you experience any symptoms of high B12 levels in blood, including :
Experts agree that proper amounts of vitamin B12 are generally safe to take . Even amounts exceeding the recommended daily amount of 2.4 micrograms usually aren’t toxic—as mentioned, what your body doesn’t absorb is normally passed as urine . In fact, in many cases, the signs and symptoms associated with high vitamin B12 levels may appear in those actively being treated for a vitamin B 12 deficiency .
All of this is to say that assessing your vitamin B12 levels through serum testing may give you peace of mind if you're concerned. If you discover they're elevated, you and your healthcare provider can investigate what's causing high B12 concentrations in your body and devise an appropriate treatment plan.
We obtain the majority of vitamin B12 we need on a daily basis from foods like [2, 9]:
However, you may need to seek out other B12-rich food sources or supplements if you:
When your nutritional needs aren’t met, you may not feel your best and brightest, to say nothing of more critical health conditions.
A vitamin B12 deficiency may result in :
On the one hand, dangerously low vitamin B12 can pose its own health risks. On the other, studies pointing to a possible association between cancer and extremely high B12 levels may give you pause .
In the end, what we know falls in line with a basic, "common sense" principle of healthcare: when kept within normal ranges, vitamin B12 is a crucially important nutrient in supporting health and well-being. Consider scheduling a blood test if you suspect your vitamin B12 level is either too high or too low, and talk with your healthcare provider about maintaining a balanced vitamin B12 concentration in your body for overall health and well-being.