How Long Does It Take For Vitamin B12 To Work?

  • By Performance Lab
  • 4 minute read
How Long Does It Take For Vitamin B12 To Work?

Vitamin B12 is a nutrient that most people don’t give a second thought to. We put it on the back burner for things like vitamin D, vitamin C, omega-3s, and more, but getting enough of this vitamin is key for optimal human performance.

It plays a major role in red blood cell health and formation and nervous system function, and low levels can spell big trouble. But if you’ve been diagnosed with a vitamin B12 deficiency and have started on supplementation, you’re probably wondering how long does it take to work?

We’re walking you through everything you need to know about vitamin B12 supplements—why B12 is important, the ins and outs of a B12 deficiency, and how long it takes before you see improvement.

Why Vitamin B12 Is Important

Vitamin B12, or cobalamin, is a water-soluble vitamin that plays an important role in overall health and well-being. While its best-known role is in erythropoiesis (red blood cell formation), it’s also involved in:

  • DNA synthesis
  • Regeneration of bone marrow, GI lining, and respiratory tract lining
  • Nervous system development and maintenance
  • Prevention of anemia

Although a B12 deficiency isn’t as common as vitamin D or even folate, it happens. It’s estimated that up to 15% of people over 60 have low levels of vitamin B12 1. But for young people, you’re not excluded from the statistics, as dietary restrictions or underlying health conditions can influence levels.

One of the other major roles B12 is involved in is the conversion of homocysteine to methionine. Upon conversion, methionine is activated into S‐adenosyl‐methionine, which plays a vital role in the nervous system and hematopoietic systems 1.

Insufficient B12 status can disrupt the homocysteine cycle and lead to accumulation, which becomes toxic neurons and poses a significant risk for vascular and cognitive health.

But here’s the thing with B12—its absorption isn’t like most other vitamins. In order for B12 to be absorbed from the gut, it requires a specific protein called intrinsic factor (IF) that is produced by parietal cells of the stomach.

Intrinsic factor forms a complex with B12 that allows it to be absorbed in the terminal portion of the small intestine 1. As a result, low levels of IF can interfere with B12 absorption and lead to a deficiency, even if dietary intake is sufficient.

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Vitamin B12 Deficiency

While a B12 deficiency may not be common in younger people, if you’re not absorbing what you’re eating or you’re not consuming enough, it could happen.

In older people, a B12 deficiency usually happens as a result of insufficient levels of intrinsic factor that limit absorption.

Low B12 can be caused by 2:

  1. Diet: B12 is found in high concentration in animal products like meat, eggs, and dairy. For people following a plant-based diet that choose to exclude animal products, it can be difficult to get enough B12. However, certain foods are fortified with vitamin B12.
  2. Pernicious anemia: This is a condition that develops when the body’s immune system attacks the parietal cells in the stomach that produce intrinsic factor. As a result, there’s not enough IF to absorb B12.
  3. Gastric surgeries: If a portion of your stomach has been removed, the body can’t produce enough IF, and the amount of space for B12 to be absorbed is also reduced.
  4. Intestinal problems: Because B12 is absorbed in the intestines, any disease that impairs intestinal function can interfere with B12 absorption.

If you think you may be low, watch out for these symptoms:

  • Fatigue and weakness
  • Loss of appetite
  • Constipation
  • Weight loss
  • Megaloblastic anemia
  • Numbness and tingling
  • Poor balance or coordination
  • Depression
  • Confusion
  • Memory loss
  • Soreness of mouth and/or tongue

How Long Before Vitamin B12 Works?

The length of time before you see the effects of vitamin B12 and symptoms subside is going to depend on a few factors:

  • B12 form
  • Administration root
  • Dosage
  • Current vitamin B12 status

Most people having B12 injections (intramuscular) will see an improvement in the first 24 hours, but the hematological response may take several days to months for a full response 3.

Health professionals recommend a full blood count and reticulocyte count 7-10 after starting treatment, and eight weeks to confirm normal blood levels.

If homocysteine or methylmalonic acid levels are abnormal, they should normalize during the first week of treatment. However, if there are neurological symptoms that have arisen due to B12 deficiency, they could take up to three months to improve, with mild improvements occurring in the first six weeks 3.

If you’re taking B12 orally, sublingual B12 is effective because it bypasses absorption in the stomach, meaning you don’t need sufficient levels of intrinsic factor to absorb it.

And studies do confirm that sublingual B12 supplementation are equally, if not more effective than intramuscular administration for correcting a B12 deficiency 4, 5.

Final Thoughts

Starting on B12 supplements is a great start if your levels are low, but don’t expect things to be back to normal immediately. Because levels are often quite low before a deficiency is recognized, it can take some time for serum concentrations to even out and symptoms to subside.

But if you’re not interested in dealing with a vitamin B12 deficiency, Performance Lab NutriGenesis Multi needs to be in your supplement stack.

It’s an ultramodern multivitamin supplying 100% DV of 17+ essential vitamins and minerals, all bioengineered with cofactors to boost absorption and bioactivities.

Find out more about Performance Lab NutriGenesis Multi here


  1. Lip GY, Lane DA, Millane TA, Tayebjee MH. Psychological interventions for depression in adolescent and adult congenital heart disease.  Cochrane Database Syst Rev. 2003;2003(3):CD004394.
  2. Oh R, Brown DL. Vitamin B12 deficiency.  Am Fam Physician. 2003;67(5):979-986.
  3. Hunt A, Harrington D, Robinson S. Vitamin B12 deficiency.  BMJ. 2014;349:g5226.
  4. Bensky MJ, Ayalon-Dangur I, Ayalon-Dangur R, et al. Comparison of sublingual vs. intramuscular administration of vitamin B12 for the treatment of patients with vitamin B12 deficiency.  Drug Deliv Transl Res. 2019;9(3):625-630.
  5. Sharabi A, Cohen E, Sulkes J, Garty M. Replacement therapy for vitamin B12 deficiency: comparison between the sublingual and oral route.  Br J Clin Pharmacol . 2003;56(6):635-638.