A systematic review concluded that the evidence suggests an association between elevated B12 and a higher risk of cancer. A scoping review of human studies concluded there is not sufficient evidence to assume that high plasma vitamin B12, high B12 intake, or treatment with pharmacological doses of vitamin B12 is causally related to cancer. Observational studies cannot show cause and effect, so this guide sets out what each source measured.
Levels, supplement use and treatment are different exposures
A blood level, the use of a supplement and a course of treatment are different things to measure. A finding about one is not a finding about the others. This table shows the exposure each source describes.
| Question | What the source says |
|---|---|
| Population-based cohort: what was measured? | Cobalamin (Cbl, vitamin B12) levels. Patients receiving Cbl treatment were excluded (abstract). |
| Dose-response meta-analysis: what was measured? | Serum vitamin B12 concentration in cohort studies (abstract). |
| VITAL cohort: what was measured? | Use of vitamin B6 and B12 from individual supplement sources and from multivitamins (abstract). |
| Scoping review: what was covered? | High plasma vitamin B12, high B12 intake, and treatment with pharmacological doses of vitamin B12 (abstract). |
| Systematic review: what was measured? | The sentence cited says elevated B12. It does not say whether that is a level, intake or treatment (abstract). |
The cohort that excluded treated patients
A population-based observational cohort study concluded that high Cbl levels were associated with the risk of subsequently diagnosed cancer, mostly within the first year of follow-up. Its abstract states that data on cancer incidence, Cbl treatment and prior diagnoses were obtained from medical registries, and that patients receiving Cbl treatment were excluded. So its finding is about levels in people not receiving treatment. It is observational and cannot show cause and effect.
What the reviews and the VITAL cohort reported
The systematic review gave risk ratios ranging 1,88 to 5,9, written with commas in the abstract. It also reported less consistent evidence linking vitamin B12 and mortality. An association does not by itself show cause and effect.
A dose-response meta-analysis of cohort studies concluded that serum vitamin B12 concentration was positively associated with the risk of all-cause mortality, especially among older adults, with a linear increasing trend. Cohort studies are observational and cannot show cause and effect. The exposure was a blood concentration, not supplement use or treatment.
The VITAL cohort reported that use of vitamin B6 and B12 from individual supplement sources, but not from multivitamins, was associated with a 30% to 40% increase in lung cancer risk among men. It is an observational cohort and cannot show cause and effect. The sentence cited concerns supplement use. It does not state a dose or an injection form.
A disclosure in the scoping review
The conflict of interest statement in the scoping review's PubMed record says the author received lectures- and consulting honorary from Worwag Pharma. It is stated here without comment on any effect, so readers can weigh it.
What the label says about overdosage
The prescribing information for cyanocobalamin injection states that no overdosage has been reported with this drug. That sentence is about the labeled product. It does not mention cancer, mortality or blood levels, and it says nothing about any compounded preparation.
What this does not tell you
None of these sources shows that B12 causes cancer or death. The scoping review's conclusion is that the evidence is not sufficient to assume a causal relationship, which is different from showing there is none. The sources do not say what a given blood level means for any one person. The cohort that excluded patients receiving treatment cannot speak to treatment, and the VITAL result is about supplement use among men. Questions about a personal blood level or treatment are for a licensed provider.