B12 As A Treatment For Peripheral Neuropathic Pain: A Systematic Review, Nutrients, August 2020

They’re talking about many kinds of pain, in various places (not a particular kind of pain, say, diabetic neuropathy, although this review included that):

According to the International Association for the Study of Pain (IASP), neuropathic pain is pain caused by a lesion or disease of the somatosensory nervous system [1]. This description applies to a highly heterogeneous spectrum of pain syndromes presenting with a broad range of signs and symptoms, with various underlying pathophysiological mechanisms.

How does B12 reduce pain?

[Vitamin B12] has been shown to act by promoting the process of myelination, leading to functional restoration [13]. It has been also suggested that B12 could promote nerve regeneration by up-regulating gene transcription [13]. B12 may decrease ectopic nerve firing, which could explain why it helps alleviate painful symptoms [14].

Findings:

In summary, there is a wealth of evidence evaluating B12 mainly as a part of combination vitamin therapies for neuropathic pain. Several of these studies show significant benefit, although they generally fall short of providing sufficient evidence to advocate for B12 supplementation due to small study sizes, short follow-up periods and absence of placebo arms. On the basis of the available evidence, the areas of strongest research supporting the use of treatments inclusive of B12 are post-herpetic neuralgia [shingles], alcohol-related neuropathy and diabetic neuropathy, but these areas along with pain caused by peripheral neuropathy of other causes, require further exploration. Currently, there are no areas in which B12 is strongly disproven.

Not really strong evidence but that was because the studies weren’t well designed (e.g. no placebo group) and not because B12 was shown not to work (“there are no areas in which B12 is strongly disproven.”).

So, biologically plausible. Some evidence. Cheap. Except for its possible effect on the colon* B12 may provide some pain relief.

* Study: Folic Acid And Vitamin B12 Supplementation Associated With Increased Risk Of Colorectal Cancer, to which a poor diet may be contributing.