This question comes up often enough to deserve a careful answer. The honest summary is that low B12 is not established as a cause of cancer, and the relationships that do exist in the literature run in more complicated directions than the question implies.
What the authoritative sources document
NIH ODS's health professional fact sheet — the most comprehensive general reference on B12 status — documents the effects of deficiency as hematological, neurological, and developmental. Its consequence list covers megaloblastic anemia, neurological damage, glossitis, fatigue, dementia, weight loss, and infertility. It does not present B12 deficiency as a cause of cancer.
Why an association can still appear
The more useful framing is that some conditions cause both low B12 and elevated cancer risk independently. NIH ODS describes pernicious anemia as an autoimmune disease causing gastric atrophy, and atrophic gastritis as a common cause of B12 malabsorption in older adults.
These are stomach conditions. A stomach condition that impairs B12 absorption is a stomach condition regardless of the B12 — so if low B12 and gastric disease appear together in data, the shared upstream condition is the more plausible explanation than B12 causing anything.
This is why "associated with" does not mean "causes." It is one of the most frequently misread distinctions in nutrition coverage.
What this means practically
Low B12 is worth investigating — not because of a cancer link, but because of what NIH ODS does establish: potentially irreversible neurological damage, and an underlying cause that is often a diagnosable gastrointestinal condition.
That second point is the real reason persistent unexplained B12 deficiency should be investigated rather than simply supplemented. The deficiency may be a signal pointing at something else.
The appropriate next step
If you have low B12, particularly without a dietary explanation, the question for a clinician is why. That investigation is genuinely valuable, and it is a clinical matter rather than something to resolve from a search result.
Our free assessment can help you describe your risk factors before that appointment.
References
This article summarizes published guidance from the sources linked above. It is general information, not medical advice, and it cannot tell you whether you personally have a deficiency. Only a clinician ordering the right test can do that. If you have symptoms that concern you, speak to a doctor.