Vitamin B12
Also called: cobalamin.
Last reviewed 2026-09-16. How this works.
When to call your team — there is nothing to watch for
Whatever you are taking or thinking of taking, the people treating you can only weigh it if they know about it. That is true when nothing has been written down here, and it is most true then.
The harm recorded on this page is a raised chance of developing cancer over years. It is not something a person can feel happening, so there are no signs to watch for, and that is the point rather than an omission. What it is a reason for is telling your team what you take.
This block is about what to watch for, and it is not all this page holds: what has been recorded about harm is below. Nothing in this block is an all-clear for a question it does not mention.
Taken through chemotherapy Grade E
Human evidence points the wrong way.
Harm has been recorded in people for this claim. The grade measures how strong the evidence for benefit is; it says nothing about the harm, and the two are read from different parts of this page.
Women who took this vitamin through chemotherapy did worse than those who did not, in one large study.
Associated with worse disease-free and overall survival when taken through chemotherapy.
The harm
Among women with breast cancer inside a cooperative group chemotherapy trial, taking vitamin B12 both before and during chemotherapy was significantly associated with poorer disease-free and overall survival, with an adjusted hazard ratio for overall survival reported as 2.04.1
In the same study, multivitamin use was not associated with poorer outcomes. The signal is specific to this one supplement rather than to supplementation in general.1
The claim
The claim is that B12 supports energy and nerve health through chemotherapy, and it is among the most commonly taken supplements during treatment.
What it does
B12 is involved in the synthesis of DNA. A supplement that supports cell division is being taken alongside treatment designed to interrupt cell division, which is the proposed explanation and is not established.1
The evidence
One limitation is central and the book states it rather than burying it. Supplement use was not randomised within that trial. People chose what they took, and people who choose a supplement differ from people who do not in ways that are hard to adjust for.1
So this is an association from observation inside a trial, not a randomised finding. It is graded E because the human evidence points the wrong way, which is this book's definition, and because the population it was found in is exactly the readership of this register.1
The studies this rests on
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Vitamin B12 use during chemotherapy within SWOG S02211
An observational study, measured survival.
B12 use before and during chemotherapy associated with poorer disease-free and overall survival.
hazard ratio 2.04. adjusted, overall survival
What is absent
No trial has randomly assigned people to take B12 through chemotherapy or not. This grade rests on an observed association, and that limitation is the main argument against it.
Scope: B12 supplementation during chemotherapy.
Established by trial scope, checked 2026-09-16. Result: supplement use in the only study in this record was self-selected rather than randomised.1
This entry is about supplementing through chemotherapy, not about treating a diagnosed deficiency. No evidence here speaks to that, and it is a clinical question for your own team.
Scope: people with a diagnosed B12 deficiency.
Established by trial scope, checked 2026-09-16. Result: the study observed supplement use in general rather than treatment of diagnosed deficiency.1
What to ask
Would you want to know that I take a B12 supplement before my next cycle?
The dish-to-person gap
The register holds no concentration comparison for vitamin B12.
Interactions
The register holds no measured interaction between vitamin B12 and a cancer treatment class. Chapter 16 of the book covers supplement interactions in general, and an empty list here means nobody has measured this one.
Nobody having looked is not the same as nothing being there. That is what this line says and all it says.
The whole interaction list, and how to turn it into a pharmacy review request.
Find a trial instead
The finding here comes from observation inside a trial rather than from a trial of B12, and the question of correcting a diagnosed deficiency is a clinical one.
References
Ambrosone CB, Zirpoli GR, Hutson AD, et al. Dietary supplement use during chemotherapy and survival outcomes of patients with breast cancer enrolled in a cooperative group clinical trial (SWOG S0221). Journal of Clinical Oncology 2020;38(8):804-814.
How this was reached: transcribed from chapter note, via docs/CHAPTER-NOTES.json, chapter 17 note 12. The author string is abbreviated in the source that was read and has not been expanded.