Vitamin B12 Benefits: What the Evidence Shows

Quick answer

Vitamin B12 benefits, in short: Vitamin B12 is an essential vitamin found in animal foods. Supplements make most sense for people who are low in B12 or are likely to be: for example, vegans, older adults, and people taking metformin [1][3][5][6]. In low-quality trials, B12 by mouth raised blood levels about as well as injections [1]. In people without a deficiency, B12 did not improve memory and thinking or mood in a pooled analysis of 16 trials [2]. We rate the evidence C: B12 raises blood levels in people who are low, but there is little evidence of other benefits for everyone else.

What is vitamin B12?

Vitamin B12 helps keep blood and nerve cells healthy and helps make DNA [5]. It is found naturally in meat, fish, milk, cheese and eggs, but plant foods contain none unless fortified [5][6]. Supplements usually contain cyanocobalamin, with methylcobalamin, adenosylcobalamin and hydroxycobalamin also sold. The US National Institutes of Health (NIH) says research hasn’t shown that any form is better than the others [5].

Official intake figures

• UK (NHS): adults aged 19 to 64 need about 1.5 micrograms (mcg) a day. Taking 2 mg (2,000 mcg) or less a day in supplements is unlikely to cause harm [6].
• US (NIH): the recommended amount for adults is 2.4 mcg a day, rising to 2.6 mcg in pregnancy and 2.8 mcg when breastfeeding [5].

Vitamin B12 benefits: what might it help with?

Possible benefit	Rating	In plain words
Raising low B12 levels	Promising	Oral and injected B12 raised levels similarly in low-quality trials [1]
Memory, thinking and mood (without deficiency)	No clear benefit	No effect in a 16-trial analysis [2]
Heart disease and stroke (via homocysteine lowering)	Mixed	No difference for heart attacks or deaths, and a small difference for stroke [4]
Energy	No clear benefit	NIH says B12 does not boost energy in people who get enough from food [5]

Ratings describe how good the research is, not a promise that it will work for you.

What the research says

• Oral versus injected B12. A Cochrane review found low-quality evidence that oral and injected B12 normalized blood levels similarly, with oral costing less. Evidence on side effects was very low quality [1]. PubMed · DOI
• Memory, mood and tiredness (16 trials, 6,276 people). In people without an advanced brain condition or overt deficiency, B12 alone or with other B vitamins had no effect on cognitive function or depressive symptoms. Too few trials measured fatigue to analyze [2]. PubMed · DOI
• Metformin (6 trials). People taking the diabetes medicine metformin had lower B12 blood levels than those on placebo or another drug (about 54 pmol/L lower), and the drop looked larger at higher doses [3]. PubMed · DOI
• Heart and stroke. A Cochrane review of homocysteine-lowering vitamins (B6, B9, B12, alone or combined) found no difference in heart attacks, deaths or side effects versus placebo, and a small difference favoring the vitamins for stroke [4]. PubMed · DOI

The key limits: much of the oral-versus-injection evidence is low quality, and trials in people with normal B12 levels don’t show extra benefits.

Amounts and how people take it

These are amounts used in studies, not recommendations.

• Typical products: supplements often provide far more than the daily need, such as 500 mcg or 1,000 mcg, but the body absorbs only a small percentage. NIH considers these doses safe [5].
• Who may need one: NIH says people over 50 should get most of their B12 from fortified foods or supplements, because many older adults struggle to absorb it from food. People who eat little or no animal food (such as vegetarians and vegans) may not get enough from diet [5]. The NHS also notes that vegans may not get enough [6].
• Medical treatment: deficiency is usually treated by a doctor, sometimes with injections [5].

Side effects, interactions and who should check first

NIH says B12 has not been shown to cause harm, even at high doses [5], while the NHS says there’s not enough evidence on very high doses and that 2 mg or less a day is unlikely to cause harm [6]. In one trial, 2 of 30 people taking oral B12 left early because of side effects [1].

Who should check with a doctor or pharmacist first

Check with a doctor or pharmacist first if you:

• take metformin, or acid-reducing medicine (such as omeprazole or lansoprazole). Both can lower B12 [3][5]
• have pernicious anemia, a stomach or bowel condition (such as celiac or Crohn’s disease), or have had stomach or bowel surgery. You may not absorb B12 normally [5]
• have symptoms of deficiency such as tiredness, numb or tingling hands and feet, or a sore tongue. Get a blood test rather than self-treating [5]
• are pregnant or breastfeeding, especially if you follow a vegan diet [5]
• take any other regular medicine or have a long-term health condition

Where to buy

Look for the active form and amount (vitamin B12 as cyanocobalamin or methylcobalamin, in mcg per serving), a stated third-party testing certificate you can check, and work out the price per daily dose rather than comparing pack prices.

FAQ

What is vitamin B12 good for?

Vitamin B12 helps keep blood and nerve cells healthy and helps make DNA [5]. As a supplement it raises blood levels in people who are low [1], but in people without a deficiency it did not improve memory, thinking or mood in a pooled analysis of 16 trials [2]. We rate the evidence C.

Does vitamin B12 work immediately?

We found no verified information on how quickly B12 works, so we can’t give a timeframe. In a Cochrane review, oral and injected B12 raised blood levels similarly in low-quality trials [1]. NIH says B12 doesn’t boost energy in people who get enough from food [5]. If you have symptoms such as tiredness or tingling, get a blood test first [5].

How much vitamin B12 per day?

The NHS figure for adults is about 1.5 mcg a day [6], and the NIH recommended amount is 2.4 mcg for most adults [5]. Supplements often provide far more, such as 500 or 1,000 mcg, but the body absorbs only a small percentage, and NIH considers these doses safe [5]. The NHS says 2 mg (2,000 mcg) or less a day is unlikely to cause harm [6].

Who should not take vitamin B12?

Check with a doctor or pharmacist first if you take metformin or acid-reducing medicine, have pernicious anemia, a stomach or bowel condition or have had stomach or bowel surgery, take any other regular medicine, or have a long-term condition [3][5]. Also check first if you are pregnant or breastfeeding, especially on a vegan diet [5].

Can you take vitamin B12 with vitamin D3 and K2?

In the sources we used for this page, we found no information on taking vitamin B12 with vitamin D or K2, so we can’t say either way. The interactions NIH lists for B12 involve metformin and acid-reducing medicines [3][5]. Ask a doctor or pharmacist before combining supplements, especially if you take any medicine.

Related pages

• Vitamin D
• Magnesium
• Omega-3
• Longevity supplements for beginners: where this fits in a beginner’s routine
• All supplements A-Z

Talk to your doctor

This page is for general information and is not medical advice. Speak to a doctor or pharmacist before starting any supplement, especially if you are pregnant or breastfeeding, take any medicine, or have a health condition.

References

We cite only studies and official pages we checked against PubMed, the publisher or the agency’s own site. The A to E evidence grades are explained in our editorial policy. See also our medical disclaimer and affiliate disclosure.

1. Wang H, Li L, Qin LL, et al. Oral vitamin B(12) versus intramuscular vitamin B(12) for vitamin B(12) deficiency. Cochrane Database Syst Rev. 2018. PubMed · DOI
2. Markun S, Gravestock I, Jäger L, et al. Effects of Vitamin B12 Supplementation on Cognitive Function, Depressive Symptoms, and Fatigue: A Systematic Review, Meta-Analysis, and Meta-Regression. Nutrients. 2021. PubMed · DOI
3. Liu Q, Li S, Quan H, et al. Vitamin B12 status in metformin treated patients: systematic review. PLoS One. 2014. PubMed · DOI
4. Martí-Carvajal AJ, Solà I, Lathyris D, et al. Homocysteine-lowering interventions for preventing cardiovascular events. Cochrane Database Syst Rev. 2017. PubMed · DOI
5. National Institutes of Health, Office of Dietary Supplements. Vitamin B12: Fact Sheet for Consumers. Updated 15 December 2023. NIH ODS
6. NHS. Vitamins and minerals: B vitamins and folic acid (vitamin B12). Page last reviewed 3 August 2020. NHS