Is vitamin B12 safe?

Upper limit

A tolerable upper intake level (UL) is the highest daily amount that is unlikely to cause harm. The US Food and Nutrition Board set no UL for B12, because B12 has a low potential for harm.^[1] In the EU, the Scientific Committee on Food also set no UL, because it found no defined adverse effects.^[2] Germany's food safety institute (BfR) found no reports of harm at about 100 µg a day from supplements. It proposes at most 25 µg per daily dose of a supplement, to leave room for fortified foods and for people who take several products.^[5]

Side effects in trials and case reports

In the Cochrane review of tablets against injections, one trial saw no side effects from treatment. In another trial, 2 of 30 people (6.7%) who took tablets left the trial early because of side effects.^[6] A review of B12 and the skin lists acne, rosacea and allergic reactions where an injection was given, and severe allergic reactions (anaphylaxis) to injections. People who are sensitive to cobalt have had skin reactions to B12 treatment.^[3]

Kidney disease from diabetes

In a 2010 trial, 238 people with kidney disease from diabetes took 1 mg of B12 with 2.5 mg of folic acid and 25 mg of vitamin B6 a day, or a placebo. After 36 months, kidney function (the filtration rate, GFR) had fallen by 16.5 mL/min/1.73 m² in the vitamin group and by 10.7 mL/min/1.73 m² in the placebo group. Heart attacks, strokes, artery procedures and deaths, counted together, were twice as common with the vitamins (hazard ratio 2.0).^[4] The trial gave 3 vitamins together, so it cannot show which one caused the harm.^[4]

Cancer

Some observational studies link high B12 blood levels or B12 supplements with cancer, and others do not. A meta-analysis of 18 trials with 74,498 people found that B vitamins, including 20 to 2,000 µg of B12 a day, had little or no effect on cancer or on deaths. NIH ODS says more research is needed.^[1]

Medicines that lower B12

Proton pump inhibitors and H2 blockers, used for heartburn and ulcers, reduce the absorption of B12 from food. Metformin, a diabetes medicine, can lower B12 levels.^[1] NICE also lists colchicine, phenobarbital, pregabalin, primidone and topiramate as risk factors for a low level.^[7]

Blood tests

A supplement can raise the B12 blood level without fully treating a deficiency. NICE advises taking the blood sample before B12 treatment starts, and asking people which B12 products they use.^[7]

Who should ask first

Talk to a doctor or pharmacist before taking B12 if you have kidney disease from diabetes, take one of the medicines above, or are pregnant or breastfeeding. The same applies if you have numbness, tingling or anemia.^[4][7]