The sources here report null results, limited data and a small difference on stroke, and they are not about the same people or the same product. A trial of cyanocobalamin injection versus placebo found no significant difference in any of the criteria measured for physical performance capacity. A Cochrane review of vitamin B found the evidence insufficient to determine whether it is beneficial or harmful for peripheral neuropathy. A methylcobalamin trial on cognition and a review on heart outcomes are covered below.
The sources side by side
Each row gives the people studied, the form, the dose, the length and the result. Where a cell says "not given here", this guide has no figure for it.
| Source | People studied | Form and dose | Length | Result |
|---|---|---|---|---|
| Trial in the British Journal of Nutrition | Thirty-one Burmese male subjects aged 18.7 to 20.8 years. Deficiency status not given here. | Injection of cyanocobalamin, 1 mg, or a placebo injection. | Not given here. | No significant difference between the two groups in any of the criteria measured, before and after administration. |
| Cochrane review | People with peripheral neuropathy, as the title describes it. Deficiency status not given here. | Vitamin B generally, rather than one specific B vitamin. Dose not given here. | Not given here. | Only limited data in randomised trials. Evidence insufficient to determine whether vitamin B is beneficial or harmful. |
| Randomized placebo-controlled trial | 271 diabetic non-demented outpatients aged 70 years or older with plasma vitamin B12 150-300 pmol/L, described as having borderline B12 status. | Methylcobalamin 1000 μg or two similar looking placebo tablets, once daily. | 27 months. | The authors concluded that vitamin B12 supplementation did not prevent cognitive decline. |
| Cochrane review | The title is about preventing cardiovascular events. Deficiency status not given here. | Supplements of vitamins B6, B9 or B12, alone or in combination, versus placebo. Dose not given here. | Not given here. | No differences on myocardial infarction, death from any cause or adverse events. A small difference in effect favouring the supplements for stroke. |
Performance and nerves
The performance trial assessed physical performance capacity before and after injection of cyanocobalamin or a placebo. It reported no significant difference between the two groups in any of the criteria measured. Because this guide has no information on whether the participants had a deficiency, the result should not be carried to people who do.
The peripheral neuropathy review is about vitamin B generally, not B12 alone. Its authors wrote that there are only limited data in randomised trials testing the efficacy of vitamin B for treating peripheral neuropathy. Their sentence covers harm as well as benefit: the evidence is insufficient to determine either.
Cognition and heart outcomes
The form studied in the cognition trial was methylcobalamin, so its result is not a result for cyanocobalamin injection. The participants were older diabetic patients with borderline vitamin B12 status, not a group described as deficient. The authors concluded that supplementation did not prevent cognitive decline in them.
The homocysteine-lowering review covers supplements of vitamins B6, B9 or B12 given alone or in combination, so it does not isolate B12. Its authors concluded that there were no differences compared with placebo on myocardial infarction, death from any cause or adverse events. For stroke, they found a small difference in effect favouring the supplements.
The review's conflict of interest statement says that in 2007 a person it names was employed by Merck to run a four-hour workshop. This guide reports that disclosure without drawing a conclusion from it.
What this does not tell you
These sources cannot tell you whether a B12 injection would affect nerve symptoms, cognition, physical performance or heart health for you. Each source covers a different form, which is cyanocobalamin injection, methylcobalamin, vitamin B generally, or B6, B9 or B12 supplements. Dose or length is missing for some, and a null result in one group or form is not a result for another. A licensed provider decides whether any of this applies to a given person.