Vitamin B12

Is vitamin B12 safe?

Yes. Vitamin B12 has one of the cleanest safety records of any supplement. Neither the US Food and Nutrition Board nor EFSA has set an upper limit, because no one has found a dose that harms healthy people. Doctors give 2,000 mcg a day by mouth, or 1,000 mcg injections, to treat pernicious anaemia without significant side effects.

The reason is absorption. The gut takes up only a couple of micrograms at a time through its normal route; the rest of a big dose crosses by passive diffusion at about 1%. A 1,000 mcg tablet delivers roughly 10 mcg, and the excess leaves in urine.

The catches are elsewhere: what is actually in the bottle, and a few warning signs from studies of long-term high doses.

How much B12 do you need?

The US recommended intake for adults is 2.4 mcg a day; the NHS puts adult needs at about 1.5 mcg. Meat, fish, eggs and dairy supply this easily. The people who genuinely need supplements are:

• Vegans and many vegetarians, because plant foods do not contain reliable B12.
• Older adults, who often absorb less B12 from food. The Linus Pauling Institute suggests 100 to 400 mcg a day for people over 50.
• People with pernicious anaemia, stomach surgery or coeliac disease, who may need injections.
• People on medicines that lower B12 (see below).

Most shop-bought products contain 500 to 5,000 mcg, hundreds of times the daily need. That is wasteful rather than dangerous for most people.

Medicines that lower your B12

B12 does not interfere with other drugs, but several common drugs reduce B12, which is why it matters for people on long-term treatment:

• Metformin: the label reports low B12 in about 7% of patients in 29-week trials, and the risk grows with years of use. Deficiency causes numbness and tingling that is easy to mistake for diabetic nerve damage.
• Proton pump inhibitors such as omeprazole, and H2 blockers: stomach acid frees B12 from food, so long-term use, usually over two to three years, can cause deficiency. B12 in tablets does not need acid and is absorbed normally.
• Colchicine, cholestyramine and some antibiotics reduce absorption from food.
• Nitrous oxide inactivates B12. Regular recreational use of "laughing gas" can cause serious nerve damage, and older patients sometimes have B12 checked before surgery.

High-dose folic acid can correct the anaemia of B12 deficiency while the nerve damage carries on unnoticed. That is why folic acid has an upper limit of 1,000 mcg and B12 does not.

Are there any risks from high doses?

Two studies give reasons not to take far more than you need.

In the US VITAL cohort of 77,118 adults aged 50 to 76 (Journal of Clinical Oncology, 2017), men who took single B12 supplements averaging more than 55 mcg a day for ten years had about twice the risk of lung cancer of non-users. The link was stronger in smokers and absent in women and in multivitamin users. It is an observational finding and does not prove B12 causes cancer, but it argues against megadoses for men who smoke.

In the DIVINe trial (JAMA, 2010), 238 people with diabetic kidney disease took a daily tablet of folic acid 2.5 mg, vitamin B6 25 mg and B12 1 mg, or a placebo. Kidney function fell faster on the vitamins, and heart attacks, strokes and deaths were about twice as common. The tablet combined three vitamins, so B12 alone cannot be blamed, but people with kidney disease should not take high-dose B vitamins without medical advice.

What is in the bottle?

This is B12's weakest area. Methylcobalamin, the form marketed as "active" B12, breaks down with light and heat. In March 2025 the supplement maker NOW published tests of 23 methylcobalamin products from lesser-known brands sold on Amazon: about half met their label claim, two contained no detectable B12 and one contained 1% of the stated amount. NOW is a competitor of the brands it tested and noted that results from an outside lab were inconsistent, so treat the exact numbers with care. The direction is clear enough. Cyanocobalamin is more stable, cheaper and the best-studied form. A USP VerifiedThird-party testingTesting of a supplement by an independent lab to check that it contains what the label says and is free of set contaminants.More in the glossary or NSF mark adds a check on dose.

How we scored it

Dose safety (92) and interactions (92) are excellent because B12 has no known toxic dose and does not alter other drugs. Regulatory record (90) and organ toxicity (88) are also strong; the DIVINe trial is the main mark against. Product quality (76) is the weakest factor because of failed label tests in methylcobalamin products. If you take B12 because of metformin or a PPI, a blood test tells you whether you need it, and vitamin C is another low-risk vitamin where most people take far more than they need.