Side effects are rare at this dose. Take one pill a day; there is no need for more.
A few people should ask a doctor first (see below).
Ready to buy:
NatureWise Vitamin B12 1000 mcg
$0.12 a day. One softgel gives 1,000 mcg, it is on NSF’s testing list, and it contains gelatin, so it is not vegan.
Vegan: Jarrow Methyl B-12 1000 mcg lemon chewable $0.13 a day.
Why 1,000 mcg is not too much
Your gut can take in only 1 to 2 mcg of B12 at a time the usual way. A small share more seeps through the gut wall, and the rest of the pill passes through you. What gets in is still several times the 2.4 mcg adults need (NIH). How B12 absorption works.
Experts left B12 without a limit because it has a low potential for harm, and the body does not store large extra amounts (NIH). That does not prove very high doses are risk-free. Few studies have followed people taking 5,000 mcg or more for years, yet about 1 in 8 of the 203 B12 labels we read hold that much.
Who should ask a doctor first
If you have kidney disease
Ask which form and dose to take. Cyanocobalamin, a common form, holds a very small amount of cyanide that healthy people clear with no trouble. Weak kidneys clear it more slowly, so one expert review suggested methylcobalamin instead. No trial has compared the two forms in kidney disease.
In one trial in diabetic kidney disease, a high-dose pill of folic acid, B6 and 1 mg of B12 was followed by faster loss of kidney function and more heart attacks, strokes and deaths than placebo. The trial could not show which vitamin was to blame (House 2010).
If your doctor prefers methylcobalamin, Thorne Vitamin B12 (methylcobalamin) is a 1,000 mcg capsule tested by NSF ($0.40 a day).
If you are a man who smokes
In a study of 77,118 adults, men who took more than 55 mcg of B12 a day from single-vitamin pills for 10 years had about twice the lung cancer risk of men who took none. The risk was highest in smokers, and women showed no link (Brasky 2017). A study like this can show a link. It cannot prove B12 was the cause.
In 18 trials with 74,498 people, 20 to 2,000 mcg of B12 a day (with other B vitamins) had little or no effect on cancer over 2 to 7 years (NIH). Still, if you are a man who smokes, ask your doctor before you take high-dose B12 for years.
If you have Leber’s disease or a cobalt allergy
Leber’s disease is an inherited disease of the eye nerve. The FDA label warns that people with early Leber’s disease who were given cyanocobalamin lost vision fast, so their eye doctor chooses the form.
Every form of B12 contains cobalt, methylcobalamin included. The FDA label lists cobalt allergy as a reason not to use it.
If you take high-dose folic acid
High-dose folic acid can hide low B12 by fixing the anemia while nerve damage goes on. If you take it, get your B12 checked too.
If you have numbness or memory changes
Get tested before you start. A pill raises your blood level and can make the cause harder to find.
Side effects people report
The best-described side effect of B12 is an acne-like rash on the face, sometimes the chest and back. In 5 reported cases it began 1 week to 5 months after starting B12 pills or shots. It cleared 3 to 6 weeks after stopping (Veraldi 2018).
Shots cause more. The FDA label for B12 shots lists itching, a short-lived rash, mild diarrhea and, rarely, a severe allergic reaction.
Some people say B12 makes them feel wired or keeps them up at night. We found no trial showing that. If you get a rash or feel unwell after you start, stop taking it and get checked.
What about 5,000 or 10,000 mcg a day?
Most people do not need more than 1,000 mcg. A 2025 review of 16 studies found no sign that bigger doses raised blood B12 more (Mazur 2025). We found no study showing that 10,000 mcg a day does more good than 1,000 mcg. For men who smoke, see the lung cancer study above.
If your doctor wants a higher dose, NOW Methyl B-12 5000 mcg lozenges hold 5,000 mcg each and are labeled vegan. Solgar Methylcobalamin 5000 mcg is a labeled-vegan cherry tablet that melts under the tongue.
Vegans have one more option. Instead of a daily pill, The Vegan Society suggests at least 2,000 mcg once a week.
Want a smaller dose? 100 mcg keeps a normal level
If your B12 level is normal and a 1,000 mcg pill feels like too much, a 100 mcg tablet a day can keep it there. It is too small to fix low B12 (Eussen 2005).
Carlyle Vitamin B12 100 mcg
$0.04 a day. Labeled vegan.
High B12 on a blood test
If you take B12, a high blood level is expected, because raising it is what the pill does. Tell your doctor you take it before the test. In someone who takes no B12, a high level can come with liver disease or some blood disorders, so doctors look for a cause.
Frequently asked questions
Can you overdose on B12?
Not in the usual sense. When B12 is injected, half or more of a 100 or 1,000 mcg shot leaves in the urine within 48 hours (FDA label). Rare reactions do happen, mostly with shots.
Can you take 1,000 mcg of B12 every day?
Yes, most adults can. It is the daily dose many trials used, and in the largest one side effects were no different from shots (Sanz-Cuesta 2020). If you take it for years, ask at your next checkup whether you still need it.
Is 1,000 mcg the same as 1,000 mg of B12?
No. 1,000 mcg (micrograms) is 1 mg (milligram). A label that says 1,000 mcg holds 1 mg of B12. A 1,000 mg dose would be 1,000 times more, and no B12 pill holds that much.
Related guides
- B12 dosage guide: how much to take a day, by age
- B12 1,000 mcg: which one to buy
- B12 deficiency signs: symptoms and the two blood tests
- Methylcobalamin vs cyanocobalamin: which form to choose
- Best B12 supplements: our full ranking
Sources
- NIH Office of Dietary Supplements. "Vitamin B12: Fact Sheet for Health Professionals" (no upper limit, absorption, cancer trials). ods.od.nih.gov (updated July 2, 2025; read October 15, 2026).
- Institute of Medicine. Dietary Reference Intakes, chapter 9: Vitamin B12. National Academies Press, 1998.
- FDA. Cyanocobalamin injection label (urine excretion, side effects, Leber’s disease, cobalt). DailyMed
- Spence JD. "B vitamin therapy for homocysteine: renal function and vitamin B12 determine cardiovascular outcomes." Clin Chem Lab Med. 2013. PMID: 23449527. House AA, et al. JAMA. 2010. PMID: 20424250
- Brasky TM, et al. "Long-term, supplemental, one-carbon metabolism-related vitamin B use in relation to lung cancer risk in the Vitamins and Lifestyle (VITAL) cohort." J Clin Oncol. 2017. PMID: 28829668
- Miller JW, et al. "Excess Folic Acid and Vitamin B12 Deficiency: Clinical Implications?" Food Nutr Bull. 2024. PMID: 38987872
- Veraldi S, et al. "Acneiform eruptions caused by vitamin B12: A report of five cases and review of the literature." J Cosmet Dermatol. 2018. PMID: 28594082
- Mazur M, et al. "Efficacy of sublingual and oral vitamin B12 versus intramuscular administration: insights from a systematic review and meta-analysis." Front Pharmacol. 2025. PMID: 41487531
- The Vegan Society. What every vegan should know about vitamin B12 (weekly 2,000 mcg route).
- Eussen SJ, et al. "Oral cyanocobalamin supplementation in older people with vitamin B12 deficiency: a dose-finding trial." Arch Intern Med. 2005. PMID: 15911731
- Ermens AA, et al. "Significance of elevated cobalamin (vitamin B12) levels in blood." Clin Biochem. 2003. PMID: 14636871. Andrès E, et al. "The pathophysiology of elevated vitamin B12 in clinical practice." QJM. 2013. PMID: 23447660
- Sanz-Cuesta T, et al. "Oral versus intramuscular administration of vitamin B12 for vitamin B12 deficiency in primary care (OB12)." BMJ Open. 2020. PMID: 32819927
- Prices are each brand’s list price, checked October 15, 2026. Amazon’s price may differ.