How long does vitamin B12 take to work?

If your B12 was already normal

Vitamin B12 does not add energy, athletic performance or endurance in people who get enough from their diet.^1,4

Most US adults get enough B12 from food, so for many people a supplement changes a lab value rather than how they feel.⁴

If your B12 was low

In trials lasting 3 to 4 months, 1,000 to 2,000 mcg of B12 a day by mouth brought low blood levels back up about as well as injections, though the evidence is of low quality.²

The Cochrane review found that none of its trials reported how people felt or their quality of life; they tracked blood levels. So these trials cannot tell you how fast you will feel different.²

What changes in blood tests

Doctors usually check B12 with a blood test, and most laboratories count results below 200 or 250 pg/mL as low. Methylmalonic acid is the most sensitive marker and is used to confirm a borderline result.⁴

In the vegan trial, both doses raised blood B12 and holotranscobalamin, another B12 marker, and lowered methylmalonic acid and homocysteine over the 90 days. Homocysteine also rises when folate runs low, so NIH calls it a less specific marker.^3,4

Why a shortage builds slowly

The body stores about 1 to 5 mg of B12, 1,000 to 2,000 times what a person usually eats in a day, so signs of running low can take years to appear.^1,4

A pooled analysis of five studies estimated that about 0.13% of the body's B12 store is lost each day.⁶

That slow drain is why someone who stops eating animal foods can go years before signs of a shortage appear.¹

Pills, under the tongue or shots

B12 dissolved under the tongue appears to work the same as B12 that is swallowed.⁴

Prescription B12 comes as an injection, usually used when a person cannot absorb B12 well, and as a nasal gel, which in one small study was absorbed at about 2%, similar to a tablet.^1,4

Shots skip the gut entirely, which is why doctors use them for people who cannot absorb B12. For people who can, high doses by mouth are an option a doctor can weigh.^1,2

When to see a doctor

Numbness, tingling, balance problems or memory changes can come from low B12, and nerve damage can occur even before anemia shows. These need testing and follow-up by a doctor, because early diagnosis is what avoids damage that cannot be reversed.^1,4

Large amounts of folic acid can mask the blood signs of low B12 while nerve damage continues.⁷

Questions people ask

Sources

1. National Institutes of Health, Office of Dietary Supplements. Vitamin B12: Fact Sheet for Consumers. Updated December 15, 2023. Opened 2026-10-07.
2. Wang H, Li L, Qin LL, et al. Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency. Cochrane Database Syst Rev. 2018;3(3):CD004655. Opened 2026-10-07.
3. Del Bo' C, Riso P, Gardana C, et al. Effect of two different sublingual dosages of vitamin B12 on cobalamin nutritional status in vegans and vegetarians with a marginal deficiency: a randomized controlled trial. Clin Nutr. 2019;38(2):575-583. Opened 2026-10-07.
4. National Institutes of Health, Office of Dietary Supplements. Vitamin B12: Fact Sheet for Health Professionals. Updated July 2, 2025. Opened 2026-10-07.
5. Eussen SJ, de Groot LC, Clarke R, et al. Oral cyanocobalamin supplementation in older people with vitamin B12 deficiency: a dose-finding trial. Arch Intern Med. 2005;165(10):1167-1172. Opened 2026-10-07.
6. Doets EL, In 't Veld PH, Szczecińska A, et al. Systematic review on daily vitamin B12 losses and bioavailability for deriving recommendations on vitamin B12 intake with the factorial approach. Ann Nutr Metab. 2013;62(4):311-322. Opened 2026-10-07.
7. National Institutes of Health, Office of Dietary Supplements. Folate: Fact Sheet for Consumers. Updated November 1, 2022. Opened 2026-10-07.