Supplements
vitamin B12 causes small-fibre neuropathy
Part of: 🧪 Vitamin B12
📅 Last reviewed: 2026-07-15 ⓘ
Evidence ladder
How far up the ladder this claim has climbed. A high consensus on a low rung means "consistent so far," not "proven in people."
Top evidence so far: Population patterns (Observational)
How the studies fall
What the evidence shows
Vitamin B12 deficiency causes small-fibre neuropathy (reduced IENFD). A competing/additive testable hypothesis for unexplained neuropathy — cheaply confirmable or excludable with B12 + MMA + homocysteine (serum B12 reads high on methyl-B12 supplementation, so MMA/homocysteine are the informative tests).
The evidence (5)
| Source | Grade | Stance | Quality | Finding |
|---|---|---|---|---|
| Güneş HN, Bekircan-Kurt CE, Tan E, Erdem-Özdamar S 2018 · Acta Neurol Belg | observational | supports | low | Pilot skin-biopsy study, n=20, both groups vitamin-B12-deficient (10 with, 10 without neuropathic pain) — there is no B12-replete control arm: 'Ten patients with vitamin B12 deficiency suffering from neuropathic pain and as control group ten patients with vitamin B12 deficiency without neuropathic pain were included.' The deficiency→small-fibre-loss signal rests on an external reference, not a control group: 'IENFD was low in two groups compared to their age normative values.' The headline contrast — 'The median of IENFD was 3.345 (1.12-5.32) in patients with neuropathic pain and 6. 20 (4.6-9.8) in controls (p < 0.001)' — separates painful from painless deficiency, not deficiency from sufficiency. Directionally supportive, self-described pilot. |
| Khan S, Zhou L 2012 · Muscle Nerve | observational | supports | low | Retrospective Cleveland Clinic case series, 63 NLD-SFSN vs 175 LD-SFSN; B12 appears only in an association tally: 'Disease associations were identified in 26 of 63 (41.3%) patients with NLD-SFSN, including ... vitamin B(12) deficiency in 2 (3.2%)'. No non-neuropathy comparator and no analysis of B12 as an exposure, so the paper cannot vote on causation. Abstract-only; prior [FT-verified] stamp not reproducible. |
| Issi 2025 · Arq Neuropsiquiatr | observational | supports | low | [FT-verified 2026-08-19] Cross-sectional case-control, 28 B12-deficient (156.9 pg/mL) vs 25 controls: MN-CSP duration shorter (40.9 vs 53.6 ms, d=-1.00), SSR foot latency longer (2.03 vs 1.85 s, d=1.07). But NO case had diagnosed SFN - all individual values sat within lab/IFCN reference limits (group-mean shift in surrogate electrophysiology); no skin biopsy or QST, exposure defined on serum B12 alone, no repletion follow-up, and the pre-specified serum-B12-vs-electrophysiology correlation was never reported. Associative only. |
| Hernández Fustes OJ et al] # backfilled 2026-08-17 from article page during sequential re-read 2026 · Aracê. | observational | supports | low | Case report, n=1 (63-year-old woman, 1 yr hand paresthesia + burning leg pain): PREP absent, serum B12 111 pg/mL; after B12 replacement, B12 rose to 304 pg/mL and PREP responses NORMALIZED — a challenge-dechallenge pattern (deficit -> repletion -> neurophysiological reversal) that supports causation at n=1. Same case previously published by the same authors as a 2025 J Clin Neurophysiol / ACNS abstract; this Aracê article is a republication. |
| de Greef, Hoeijmakers, Gorissen-Brouwers, Geerts, Faber, Merkies 2018 · Eur J Neurol | observational | mixed | low | [FT-verified 2026-08-19] Both-ways within one paper. Supporting side: 'Vitamin B12 deficiency was present in 4.7% of SFN patients.' and the authors conclude 'The prevalence of vitamin B12 deficiency in our SFN population was higher than in the general population.' Against side: the comparator is unmatched literature prevalence which the same paragraph says 'increases gradually up to 10% or higher in people of 70 years or older' - at this cohort's median age of 53 (IQR 44-61.5) the 4.7% falls INSIDE that band, and no age-adjusted comparison or P value is reported; more decisively, among the 696 patients without a known comorbidity at presentation - the only group where screening actually tests enrichment - the workup found 'vitamin B12 deficiency in 0.75% and MGUS in 0.6%', i.e. below general-population background, so the headline 4.7% is largely referral of patients already carrying the diagnosis. Held at mixed rather than contradicts because 'Homocysteine or methylmalonic acid was not assessed', which the authors concede biases ascertainment downward. No repletion/reversal data - the paper measures association only ('For these conditions a causal relationship with small nerve fiber damage is suspected.'). |
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