How long does vitamin B12 take to work?

Updated 3 Nov 2026

Short answer

In people with a deficiency, symptoms may start to improve within 2 weeks of treatment, but this can take up to 3 months, and longer to go away fully.^[1] In a trial of high-dose tablets and injections, blood counts and nerve problems got better quickly in both groups.^[2] In people without a clear deficiency, trials found no effect on memory or mood.^[3]

Sources (3)

Vitamin B12 deficiency in over 16s: diagnosis and management (NG239). National Institute for Health and Care Excellence (NICE), 2024. www.nice.org.uk
Effective treatment of cobalamin deficiency with oral cobalamin. Kuzminski AM, Del Giacco EJ, Allen RH, Stabler SP, Lindenbaum J Blood, 1998. PMID 9694707
Effects of Vitamin B12 Supplementation on Cognitive Function, Depressive Symptoms, and Fatigue: A Systematic Review, Meta-Analysis, and Meta-Regression. Markun S, Gravestock I, Jäger L, Rosemann T, Pichierri G, Burgstaller JM Nutrients, 2021. PMID 33809274

More about vitamin b12

What is vitamin B12?Who is likely to be low in vitamin B12?What is vitamin B12 used for?Which foods contain vitamin B12?Is vitamin B12 safe?Does vitamin B12 give you more energy?

For education; not medical advice. Talk to a doctor before you start, stop or change a supplement.