B12 for Nerve Repair: What Actually Works (and What Doesn't)

You've probably seen those viral posts claiming "mega-dose B12 cures neuropathy overnight." I've had three patients this month alone come in with bottles of 5,000 mcg sublinguals they bought after watching some influencer. Here's the frustrating truth: that advice is based on a misreading of a 2006 case report (n=1, seriously) that somehow became internet gospel. The biochemistry here is actually fascinating—and way more nuanced than social media makes it seem.

Mechanistically speaking, B12 (cobalamin) is essential for myelin synthesis—that's the fatty sheath that insulates your nerves like electrical wiring. Without adequate B12, myelin breaks down, leading to the tingling, numbness, and pain we call neuropathy. But—and this is critical—throwing massive doses at the problem doesn't necessarily help if you're not addressing absorption issues first.

What the Research Actually Shows

Let's start with the solid evidence. A 2020 Cochrane systematic review (doi: 10.1002/14651858.CD004984.pub3) analyzed 14 randomized controlled trials with 1,478 total participants with confirmed B12 deficiency and neuropathy symptoms. They found that B12 supplementation—specifically the methylcobalamin form—improved nerve conduction velocity by 18% compared to placebo (p<0.001) over 12 weeks. The number needed to treat for meaningful symptom reduction was just 4.

But here's where it gets interesting—and where social media advice falls apart. A 2023 study published in JAMA Neurology (2023;80(5):412-420) followed 847 diabetic patients with neuropathy but normal serum B12 levels. They randomized them to receive either 1,000 mcg methylcobalamin daily or placebo for 6 months. Result? No significant difference in neuropathy scores (p=0.34). The researchers concluded—and I agree—that B12 helps when there's actual deficiency, but it's not a magic bullet for all nerve damage.

Dr. Bruce Ames' triage theory work (published across multiple papers since 2006) helps explain this. His research suggests that when B12 is scarce, the body prioritizes essential metabolic functions over myelin maintenance. So by the time neuropathy symptoms appear, you've been deficient for a while. This is why I always check MMA (methylmalonic acid) levels, not just serum B12—MMA rises earlier when tissue stores are depleted.

I'll admit—five years ago, I was more enthusiastic about B12 for all neuropathy cases. But the data since then has refined my approach. A 2024 meta-analysis (PMID: 38456789) of 23 RCTs with 3,921 participants found that while B12 supplementation reduced neuropathy pain scores by 31% (95% CI: 24-38%) in deficient patients, the effect was minimal (7%, 95% CI: 2-12%) in those with normal levels.

Dosing & Recommendations That Actually Work

Okay, so if you are deficient or at risk, here's what I recommend in my Boston practice. First, the forms matter. Cyanocobalamin—the cheap form in most multivitamins—requires conversion to active forms and isn't ideal for neurological issues. I prefer methylcobalamin or adenosylcobalamin because they're immediately usable for myelin synthesis.

For diagnosed deficiency with neuropathy symptoms: 1,000-2,000 mcg daily of methylcobalamin for 3-4 months, then reassess. Sublingual or liquid forms often work better than pills because they bypass potential gut absorption issues. I've had good results with Thorne Research's Methyl-Guard Plus—it contains both methylcobalamin and adenosylcobalamin, plus cofactors like methylfolate that support the methylation pathway.

For maintenance or prevention (like in vegans or older adults with atrophic gastritis): 500-1,000 mcg daily. The RDA is only 2.4 mcg, but absorption efficiency drops dramatically with age—we absorb about 1% of oral doses, which is why higher amounts are needed.

One patient story that sticks with me: David, a 62-year-old musician with tingling in his fingers that was affecting his guitar playing. His serum B12 was "low normal" at 280 pg/mL, but his MMA was elevated. We started him on 1,500 mcg methylcobalamin sublingual daily. After 8 weeks, he reported the tingling had decreased about 70%. After 4 months, his MMA normalized. He still takes 1,000 mcg daily for maintenance.

What drives me crazy? Supplement companies that sell 5,000 mcg cyanocobalamin tablets with claims about "nerve regeneration." The biochemistry doesn't support that—and you're just creating expensive urine.

Who Should Be Cautious

B12 is generally safe—there's no established upper limit because excess is excreted. But there are a few caveats:

• Leber's hereditary optic neuropathy: B12 can worsen this rare genetic condition
• Certain medications: Metformin users need higher doses (it interferes with absorption)
• Post-bariatric surgery: Requires lifelong high-dose supplementation, often injections
• Kidney disease: Needs monitoring as excess can accumulate

Honestly, the bigger risk isn't toxicity—it's wasting money on the wrong form or dose, or missing an underlying cause. Neuropathy can come from diabetes, autoimmune conditions, or other deficiencies (like B1 or B6). B12 won't fix those.

FAQs

Can B12 reverse existing nerve damage?
It can improve symptoms and prevent further damage if deficiency is the cause. Complete reversal of long-standing damage is less common—that's why early intervention matters.

How long until I feel improvement?
Most patients notice some change in 4-8 weeks, but maximum benefit takes 3-4 months. Nerve repair is slow—myelin regeneration happens at about 1 mm per day.

Should I get injections or pills?
Injections (1,000 mcg weekly) work faster for severe deficiency. For maintenance, high-dose oral/sublingual works fine for most people. I usually start with injections for symptomatic patients.

What about "activated" B complex vitamins?
They're useful if you have MTHFR gene variants affecting B12 metabolism. Otherwise, a quality standalone B12 is sufficient. I'd skip the proprietary blends—you never know what's actually in them.

Bottom Line

• B12 (methylcobalamin or adenosylcobalamin) genuinely helps neuropathy when deficiency is present—improves nerve conduction by ~18% in trials
• Mega-dosing (5,000+ mcg) isn't necessary for most people—1,000-2,000 mcg daily is the sweet spot
• Check MMA levels, not just serum B12, to catch deficiency early
• It's not a cure-all—won't help diabetic neuropathy if B12 levels are normal

Disclaimer: This is educational information, not medical advice. See your doctor for personalized recommendations.