Quick answer
Vitamin B12 has not been shown to cause harm, even at high doses, and no upper limit is set. The side effect most often described is acne-like breakouts after high doses, mostly in case reports, which generally cleared after stopping. Most of a high-dose tablet is never absorbed.1,2,3
Key facts
- NIH says doses much higher than the recommended amount, such as 500 or 1,000 mcg, are considered safe.1
- The medicines known to interact with B12, metformin and acid reducers, lower it rather than add to it.3
- Only about 2% of a 500 mcg dose is absorbed.3
Can you take too much?
No upper limit is set for vitamin B12 because it has a low potential for harm. NIH says it is generally considered safe even at large doses, such as 500 or 1,000 mcg, because the body does not store the excess.1,3
Because so little of a large dose is absorbed, most of a high-dose tablet never reaches the blood: about 1.3% of 1,000 mcg is taken up.3
Side effects on the skin
Acne-like skin eruptions have been reported with high-dose B12 supplements, mostly in case reports, and they generally clear after the supplement is stopped. In one small study, 1 of 10 healthy people given extra B12 developed acne a week later.2,4
A 2020 review lists high-dose B6 and B12 among the supplements linked to acne and describes the breakouts as uniform bumps. Because they generally clear once the supplement is stopped, stopping it and telling your doctor is a sensible first step if breakouts start after you begin B12.2
High B12 and cancer
Studies of high B12 and cancer disagree. In one cohort of 77,118 people, men who took at least 55 mcg a day from supplements for about 10 years had a 40% higher risk of lung cancer, with no link in women, while a review of 18 trials in 74,498 people found that B vitamin supplements with 20 to 2,000 mcg of B12 had little or no effect on cancer or deaths.3
In a large observational analysis, people with blood B12 above 813 pg/mL had a 1.74 to 4.72 times higher 1-year risk of a cancer diagnosis than people in the normal range. Studies of this kind cannot show that B12 causes cancer.3
NIH concludes that more research is needed to know whether high or low B12 intakes affect cancer risk.3
Side effects for men and for women
In the cohort study, the higher lung cancer risk was seen only in men; women in the same study showed no link. The sources we use report no other side effects that differ by sex.3
Recommended amounts rise slightly in pregnancy and breastfeeding, to 2.6 and 2.8 mcg a day.1
Medicines that lower B12
Your doctor may want to check on you
Metformin may lower vitamin B12 levels, possibly by interfering with its absorption. In 29-week trials described on the metformin label, about 7% of people with normal B12 dropped below normal, and the label tells prescribers to measure B12 every 2 to 3 years.1,3,5,6
On the medicine's label
Your doctor may want to check on you
Proton pump inhibitors such as omeprazole and lansoprazole reduce stomach acid, which the body needs to release B12 from food. The omeprazole label says daily use of any acid-suppressing medicine for a long time, such as more than 3 years, may lead to poor absorption of B12.1,3,7
On the medicine's label
Taking B12 every day
The body loses a small share of its B12 store each day, estimated at 0.13% in one pooled analysis, so a steady intake from food or supplements keeps stores level.8
Who should ask first
- Anyone taking metformin, a proton pump inhibitor or an H2 blocker
- Anyone with numbness, tingling, balance problems or memory changes, which need testing
- Anyone with pernicious anemia or past stomach or weight-loss surgery, who may need shots or high doses under medical care1
- Anyone pregnant or breastfeeding who eats no animal foods
Questions people ask
Is it bad to take B12 if you are not deficient?
No harm has been shown, but there is also no gain in energy or performance for people who already get enough.1
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Sources
- National Institutes of Health, Office of Dietary Supplements. Vitamin B12: Fact Sheet for Consumers. Updated December 15, 2023. Opened 2026-10-07.
- Zamil DH, Perez-Sanchez A, Katta R. Acne related to dietary supplements. Dermatol Online J. 2020;26(8):13030/qt9rp7t2p2. Opened 2026-10-07.
- National Institutes of Health, Office of Dietary Supplements. Vitamin B12: Fact Sheet for Health Professionals. Updated July 2, 2025. Opened 2026-10-07.
- Kang D, Shi B, Erfe MC, Craft N, Li H. Vitamin B12 modulates the transcriptome of the skin microbiota in acne pathogenesis. Sci Transl Med. 2015;7(293):293ra103. Opened 2026-10-07.
- Metformin hydrochloride tablets and extended-release tablets: prescribing information (Laurus Labs). DailyMed, U.S. National Library of Medicine. Published September 28, 2026. Opened 2026-10-07.
- MedlinePlus. Metformin. U.S. National Library of Medicine. Last revised February 15, 2024. Opened 2026-10-07.
- Omeprazole delayed-release capsules: prescribing information (Preferred Pharmaceuticals). DailyMed, U.S. National Library of Medicine. Published September 28, 2026. Opened 2026-10-07.
- 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.
This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.