Vitamin B12 Overdose


Vitamin B12 Overdose

Find out why a vitamin B12 overdose is almost impossible and is actually often even recommended for B12 treatments and therapy.

Is a Vitamin B12 Overdose Possible?


  • An overdose of vitamin B12 is almost impossible
  • The only known side effect is a form of acne, but this is very rare
  • In therapy, a higher dose of vitamin B12 becomes more useful, as the body can only utilize part of it
  • Doses of 500-2,000 µg per day are normal for treating a vitamin B12 deficiency
  • Oral supplements are just as effective as injections

Many people express concerns regarding the possibility of a vitamin B12 overdose as a result of the high doses often used in many vitamin B12 supplements. This is with good reason, as it would indeed be very risky to interfere with the body’s metabolism for many other vitamins. However, is it really possible to overdose on vitamin B12?

Fortunately not. Overdosing on vitamin B12 is almost impossible, since the body simply excretes the unnecessary extra amounts. However, it is also important not to overdo it, as this excess amount is excreted through the kidneys; representing a moderate, albeit typically manageable burden.

In general, it is typically recommended to cover the body’s daily requirement by taking a small dosage of vitamin B12 daily,  rather than taking huge quantities of B12 ‘in advance’. Even for those suffering from malabsorption, an oral dose of around 500 µg will typically suffice for covering the RDA – higher doses are only really necessary in the therapeutic milieu.

Vitamin B12 Oversupply

The daily requirement of vitamin B12 is typically given as around 3 µg for adults, and the diet alone can in some cases deliver a big oversupply: 100g of calf’s liver provides an astonishing 60 µg of vitamin B12 – 2000% of the daily requirement. Certain modern vitamin B12 supplements even provide as much as 5000 µg per dose – representing around 160,000 percent of the RDA.

At first glance these quantities seem gigantic, yet they are deceptive since the amount supplied through the diet and supplements does not correspond to the actual amount absorbed. As we will explore further below, only a small quantity of the vitamin B12 supplied through nutrition can actually be absorbed by the body. As a result of this, an overdose is actually urgently necessary for those with malabsorption, in order to ensure the body has an adequate supply of vitamin B12.

Recent research suggests that the limits for a B12 deficiency and the recommended daily amount could well be set too low, meaning that the exact figures for an oversupply of vitamin B12 are not exactly straightforward.

Vitamin B12 Overdose: Symptoms and Side Effects

Vitamin B12 is very well researched today and even after decades of studies, there are still almost no known symptoms of a vitamin B12 overdose, even with extremely high doses. As a result of this, no maximum dose of vitamin B12 has been put in place.  The aforementioned excretion issues aside, it is theoretically possible to safely take even the largest amounts of vitamin B12 without fear of a negative effect.

The only active ingredient which poses any slight risk at all is cyanocobalamin, since this releases a tiny amount of cyanide within the body, which some people can react negatively to. This amount is so tiny, however, that it is considered medically irrelevant in usual doses.

In individual cases, intramuscular injections of higher doses of vitamin B12 can trigger slight defense responses in the body, such as skin irritations and a special form of acne. Hot flushes, dizziness and nausea are also occasionally reported, but these are typically attributed to the preservatives added to the supplement, as opposed to the B12 itself.

Vitamin B12 and Acne

For some patients, particularly high dosages of vitamin B12 injections can lead to outbreaks of acne, as has been shown in various studies. The cause of this occurrence has unfortunately not been fully researched. The acne clears up, however, as soon as the vitamin B12 supplement ceases to be used.1,2,3

At present, it remains unclear as to which forms of vitamin B12 and in which doses cause this effect. Given that the side effect has been seen to occur after doses of as little as 20 µg, it appears that the threshold differs from person to person.

This acne is no indicator of an overdose. In most cases, it is likely that a cobalt allergy is the cause of the reaction – cobalt is the central atom in vitamin B12 (which is where ‘cobalamin’ stems from). It is therefore possible that the acne occurrence after large amounts of vitamin B12 is as a result of a cobalt allergy.

Vitamin B12 Overdose and Pregnancy

According to current scientific knowledge, pregnant women have absolutely no worries regarding a possible vitamin B12 overdose. This is because the unused vitamin B12 is simply excreted in the urine, meaning it does not travel along the bloodstream and subsequently cannot reach the foetus. Those who are expecting should actually be concerned with ensuring an adequate supply of vitamin B12 and folic acid, since a deficiency in either can seriously endanger the foetus. For both pregnant women and those who are breastfeeding, an increased RDA of around 4- 6 µg of vitamin B12 should be taken, to ensure that the vitamin B12 supply remains sufficient in this time period.

Despite this, doses of over 500 µg are just as similarly ill-advised in these cases.

Vitamin B12: High or Low Dosage?

B12 deficiency therapy indicates that, when used orally, only vitamin B12 supplements with a dose of around 500 µg per day leads to significant improvements within the relative markers for patients with severe intake disorders.4,5 Dosages of  500 – 2000 µg per day have long been the standard practice in vitamin B12 deficiency therapy for diseases.

This is easy to understand once the metabolism of vitamin B12 is visualised. Vitamin B12 is absorbed through the help of a special molecule, the intrinsic factor (IF). The maximum vitamin B12 that a body with a healthy metabolism can absorb is around just 1.5 µg per dose via the IF. A further 1% of the dose can be additionally absorbed through passive diffusion in the bloodstream.

A dose of 500 µg of vitamin B12 can thus only yield a maximum of 1.5 µg absorbed through the IF and an additional 5 µg absorbed through passive diffusion, illustrating that the overdose is nowhere near as high as it appears at first. Any lower dosage can even lead to a vitamin B12 shortage for sufferers of intake disorders. As an example, let’s take a supplement with 50 µg: the 1.5 µg absorbed through the IF disappears almost immediately amongst those with absorption difficulties, leaving the body with just the 0.5 µg absorbed via passive diffusion – so despite the overdose, this represents just a fraction of the RDA.

Since most people with a vitamin B12 deficiency suffer as a result of an intake disorder (their body cannot correctly utilize the vitamin B12), they are entirely reliant on the small amount of B12 which can be absorbed through passive diffusion. Thus it is clear that such a high overdose of vitamin B12 is important and necessary to achieve the desired effect.

Overdose of a Vitamin B Complex

When taking vitamin B complexes, it is worth adopting caution regarding vitamin B5 and B6. An overdose of vitamin B5 can cause vasodilation, itchiness, nausea, headaches and allergies. An intake of grams of B5 can even lead to permanent liver damage and inflammation of the stomach lining. Vitamin B6 is considered harmless in usual amounts, but doses higher than 2 g can cause movement or nerve damage.

Vitamin B12 Overdose for Detoxification

There are other cases in which an extreme overdose of vitamin B12 is important for a successful treatment. Very high doses of hydroxycobalamin are prescribed when treating an intoxication of cyanide, because the vitamin binds with the cyanide and channels it.6

Ultra high doses of methylcobalamin have also been proven in testing on rats as well as people to be effective in the regeneration of nerves.7,8

Vitamin B12 Overdose for Oral Intake

For a long time, it was the view of therapists that high doses of vitamin B12 had to be injected intramuscularly to be particularly effective. However, most researchers take the view today that an oral intake of a high dose vitamin B12 supplement can be just as effective as a monthly injection.9

Vitamin B12 supplements like pills, capsules or drops can also be seen as just as effective as vitamin B12 shots, when the corresponding overdoses are taken.

If no intake disorder exists, daily oral doses of 150 – 250 µg are recommended to suffice. For known malabsorption, the supplement should contain at least 500 µg in order to be sure of covering a deficiency.

Since an overdose is almost impossible, the current trend amongst producers is to simply only offer their vitamin B12 supplements at very high dosages. Such supplements can in principle be taken by all people without any risk. However, a dosage of more than the body’s requirement is at best merely useless, which is why dosages of over 500 µg are being critically examined.

At the same time, many naturopaths recommend a daily intake as opposed to a larger monthly or weekly dose, which cause the B12 levels in the blood to rise extremely high for a short time. Dosages in the region of 250 – 500 µg should be enough to cover the body’s daily requirement in almost all cases.


1 Jansen T, Romiti R, Kreuter A, Altmeyer P. Rosacea fulminans triggered by high-dose vitamins B6 and B12. J Eur Acad Dermatol Venereol. 2001 Sep;15(5):484-5.
2 Sherertz EF. Acneiform eruption due to “megadose” vitamins B6 and B12. Cutis. 1991 Aug;48(2):119-20.  (Abstract)
3 Braun-Falco O, Lincke H. The problem of vitamin B6/B12 acne. A contribution on acne medicamentosa. MMW Munch Med Wochenschr. 1976 Feb 6;118(6):155-60. (Abstract)
4 Eussen SM, de Groot LM, Clarke R, et al.Oral Cyanocobalamin Supplementation in Older People With Vitamin B12 Deficiency: A Dose-Finding Trial.Arch Intern Med. 2005;165(10):1167-1172. doi:10.1001/archinte.165.10.1167.
5 Rajan, S., Wallace, J. I., Brodkin, K. I., Beresford, S. A., Allen, R. H. and Stabler, S. P. (2002), Response of Elevated Methylmalonic Acid to Three Dose Levels of Oral Cobalamin in Older Adults. Journal of the American Geriatrics Society, 50: 1789–1795. doi: 10.1046/j.1532-5415.2002.50506.x
6 John P. Thompson, Timothy C. Marrs Hydroxocobalamin in cyanide poisoning December 2012, Vol. 50, No. 10 , Pages 875-885 (doi:10.3109/15563650.2012.742197)
7 Kuwabara S, Nakazawa R, Azuma N, Suzuki M, Miyajima K, Fukutake T, Hattori T Intravenous methylcobalamin treatment for uremic and diabetic neuropathy in chronic hemodialysis patients. In: Intern Med (1999 Jun) 38(6):472-5
8 Tetsuya Watanabe, Ryuji Kaji, Nobuyuki Oka, William Bara, Jun Kimura, Ultra-high dose methylcobalamin promotes nerve regeneration in experimental acrylamide neuropathy, Journal of the Neurological Sciences, Volume 122, Issue 2, April 1994, Pages 140-143, ISSN 0022-510X,
9 Butler CC, Vidal-Alaball J, Cannings-John R, McCaddon A, Hood K, Papaioannou A, Mcdowell I, Goringe A. Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency: a systematic review of randomized controlled trials. Fam Pract. 2006 Jun;23(3):279-85. Epub 2006 Apr 3. Review. PubMed PMID: 16585128.

Leave a Reply