Ill Sick Allergic Man

Vitamin B12 Overdose And Side Effects

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The Internet is littered with tales of people claiming to have overdosed on vitamin B12, or saying that they suffered various unpleasant symptoms as a result of taking B12 supplements. But the fact is, vitamin B12 is not toxic. If you’re an average, healthy person, you don’t need to worry about negative consequences from taking B12.

Vitamin B12 Is Not Toxic
Some vitamins are dangerous in large quantities. For example, too much vitamin A will kill you (which is why you should never eat polar bear liver!), and high concentrations of vitamins D, E and K are also harmful. But these vitamins are fat-soluble, so they build up inside your body. Vitamin B12 is not fat-soluble; it’s water-soluble instead. So any excess B12 in your body is simply excreted out in urine.

Scientist ResearchVitamin B12 Overdose
There have been various scientific studies where test subjects were given extremely high doses of vitamin B12 (often several thousand times the RDA) for extended periods of time (months or years). None of this research suggested that these large quantities of B12 were in any way dangerous – there were no adverse effects.

Hydroxocobalamin (a vitamer of B12) is used to treat cyanide poisoning, most commonly as a result of smoke inhalation. This is generally done by intravenously administering 5mg of B12 (the RDA is 2.4mcg!), and the procedure is sometimes repeated less than an hour afterwards. There would be no reason for anyone to ever take more than this amount of B12, and it doesn’t harm the patients – in fact it often saves their lives.

Safe Not DangerousMany national health authorities don’t bother giving an
upper limit to the amount of vitamin B12 that is safe to take, because as far as anyone knows, there is no upper limit. The US Food and Nutrition Board is just one of many national organisations that take this approach.

The quantities of B12 found in food are tiny compared to what you can find in B12 supplements, so there’s certainly no risk of overdosing from your diet alone.

Basically, unless you take more B12 than scientists have ever even considered, you won’t overdose. And even if you did take more, you’d probably still be fine.


Vitamin B12 Allergies
Despite not being toxic, it is possible to be allergic to vitamin B12 (or cobalt, a key building block of B12). However, it should be pointed out that vitamin allergies are exceptionally rare. If you have an allergic reaction to a dietary supplement, the chances are that it’s not a vitamin causing the problem, but a different ingredient. Either way, you should seek treatment immediately.

For the unfortunate few who are allergic to B12, symptoms may include:

  • Ill Sick Allergic ManNumbness or tingling in the hands, arms and face
  • Skin complaints e.g. acne, eczema or a rash
  • Nausea
  • Difficulty swallowing
  • Diarrhea
  • Anaphylactic shock (very rarely)

Pre-Existing Medical Conditions
Vitamin B12 is known to interact with a number of different medications – you can take a look at our article on medical interactions for more information.

You should also be wary of taking B12 supplements if you suffer from any of these conditions:

  • Leber’s disease – Taking B12 while suffering from Leber’s disease can seriously damage the optic nerve, potentially resulting in blindness.
  • Gout – Individuals who are susceptible to gout may have it triggered by B12 supplements.
  • High blood pressure – There have been reports of temporary increases in blood pressure after intravenous B12 administration.
  • Megaloblastic anemia – Treatment with B12 can sometimes reduce potassium levels to the point where potentially fatal hypokalemia sets in.
  • Stents – Vitamin B12 supplements can lead to a narrowing of the blood vessels after stenting.

Vitamin B12 SupplementsThe B12 Deficiency Alternative
The point is that you shouldn’t be put off taking B12 supplements by the risk of
overdose, toxicity or allergic reactions. Unless you fall into one of the very small and well-defined categories outlined above, you have absolutely nothing to fear.

For the vast majority of people, it would be far better to take more than the RDA of vitamin B12 than to take too little. Not getting enough B12 can lead to vitamin B12 deficiency. This is a serious and unpleasant illness, and everyone should try to minimise their risk of suffering from it. Unfounded fears do nothing but stop people from taking supplements that would probably make them healthier and happier in the long run.

If you’d like some advice on choosing a supplement, please take a look at our Choosing a B12 Supplement and Recommended B12 Supplements pages. Alternatively, check out the Amazon products below to see what’s popular with our readers right now!


From top to bottom, images are courtesy of cooldesign, imagerymajestic, Mister GC, and jk1991, all at FreeDigitalPhotos.net.

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27 thoughts on “Vitamin B12 Overdose And Side Effects”

  1. I have been injecting some vitamins from my bioidentical doctor. I had some blood work done. My results were my vitaminB-12 was2402 high. The test said normal is 211-911 . Is this level dangerous? Also I have been very bloated and constapated

    1. Hi Melody, apologies for the delay – we’ve been suffering some technical difficulties which we’ve only just been able to fully resolve. Unfortunately that means this reply is probably too late, but I’ll respond anyway in case someone else comes along with a similar question.

      That is certainly a high level, which is unusual as excess B12 is normally excreted out in the urine. It’s unlikely to do you any harm in itself, and it’s much less of a worry than low B12 levels would be! If you’ve only been taking the vitamins prescribed by your doctor, then I wouldn’t worry about it at all. If you’re still concerned, he or she is the one you should talk to – no online advice from someone who hasn’t examined you will be as good as personal care from someone who knows you and your history.

      I hope that helps!

      1. I’m not a doctor, but noticed she said she was bloated at the time… meaning she’s retaining water. If she’s retaining, she may not be urinating enough to excrete it, and thus it’s building up.

        But if it’s non-toxic, then there’s no worry. (But if the bladder’s not working to excrete, then excess B12 is the least of your worries.)

        – Jeff

  2. Melody,

    Have you been tested for the mthfr gene mutation? I have always had abnormally high b12 & a few other issues & have recently learned I have this mutation which affects folate & b12 levels. I was reading this article as I’ve ended up in hospital with heart problems since I’ve started taking methylcobalamin & methyl folate for the gene mutation. I suspect there is a chain reaction going on which can make b12 harmful if the underlying issues in people with this mutation are not addressed. It’s believed the mutation is also very common. Hope this helps.

    1. Hi Jenny! I am mthfr 677 heterzygous and i was supplementing methyilfolate &methylcobalamin for 8 months and then paused for 3 months when i got tested…my levels were hight 976…in our clinic up to 600 was normal. I didnt really understand your post…did you mean that even though we are supplementing the right active form of b vitamines and folate, problem with methyilation can leade to high levels in blood?
      i’m pretty confued what should i do now…i will start to prepare form my pregnancy soon and I am not sure should i supplement b12…i am scared to risk anything since i had two missed miscarriages. 🙁

      1. Hi Diana, sorry to be confusing. What I was trying to say is that I have always had extremely high b12 blood levels and excellent folate labels. Doctors have always told me I’m really healthy & have no issues with either of these. Multiple miscarriages have sent me in a research journey the past few years. I have since discovered that whilst my blood levels of these are high – my cell levels of these are dangerously low. Doctors don’t test your cell levels. I’ve since learned I’m a double C677T and that b12 & folate sit in my blood while my cells are completely starved. I’m not getting proper treatment which includes supplements of these in their methylated forms. When I started the supplements the kick start was too powerful for me & caused heart issues. 40 years of living on 20% of these levels in my cells is taking a bit of time to recover from so I need to go slow. If someone like me takes methylated b12 /folate without being aware of an mthfr mutation they could be endangering themselves.

        1. Hi Diana, sorry to be confusing. What I was trying to say is that I have always had extremely high b12 blood levels and excellent folate levels. Doctors have always told me I’m really healthy & have no issues with either of these. Multiple miscarriages have sent me in a research journey the past few years. I have since discovered that whilst my blood levels of these are high – my cell levels of these are dangerously low. Doctors don’t test your cell levels. I’ve since learned I’m a double C677T and that b12 & folate sit in my blood while my cells are completely starved. I’m now getting proper treatment which includes supplements of these in their methylated forms. When I started the supplements the kick start was too powerful for me & caused heart issues. 40 years of living on 20% of these levels in my cells is taking a bit of time to recover from so I need to go slow. If someone like me takes methylated b12 /folate without being aware of an mthfr mutation they could be endangering themselves.

          1. Sorry for the duplication. As a ps I’m planning a pregnancy in the next few months also & am under the care of mthfr support Australia who specialise in mthfr & pregnancy. You do need these supplements for a healthy pregnancy but you need to go slow and incrementally dose up over a period of time.

          2. Thank you for your answer. I am mthfr C677T heterozygous and i’ve started with 800 mcg of methyilfolate and 1000 mcg of b12 methyilcobalamin and b6 P5P 50 mg at the begining, then when i discovered i am pregnant i’ve took higher dosed…1000 mcg of methyilfolate and 5000 mcg of methyilcobalamin…my bloodwork was great MCH, MCHC, MCV, RDW, thrombocites, haemoglobine, erithrocites and leukocytes….than i had miscarriage…and 3 month later without supplement i tested hight b12 and low homocysteine…i think that maybe i was over methyilated because of low homocyteine levels. WHat do you suggest? how slow should I start again with methyilcobalamin? 600 mcg?
            the thinng is that i felt better when i was supplementing, i didnt felt overmethyilated…also i do have gut inflamationt that i recently discovered and i think that’s why i dont get enough b12 in cell and why it stays in blood

          3. Hi Diana
            Do you have the care of a natropath or functional medicine practitioner speciaalising in MTHFR? If not, I strongly recommend it as I have no training in this. I can only tell you about my experience. I have devoured Ben Lynch’s information on MTHFR.net From what I can see, 5,000 is possibly too much methylcobalamin for a single mutation – I have a double and I have been recommended to aim to get up to 2,000 per day. What I have been advised is to take the Thorne research Prenatal supplement. Then on top of that, aim to get another 800 L5mfolate and 1,000 methcobalamin. I have a whole lot of other stuff also. Make sure you are taking the right form of folate and cobalamin and avoid folic acid like the plague. Where I live folic acid is added to just about everything so I have to be very careful what I eat. I also have low homocystine but recently did a methylation panel which showed I had a very high amount of SAH (which I believe is the precursor to homosystine). Apparently, that is meant to be converted into homocystine but there is a chemical reaction involving magnesium that does it. I also had super high levels of folinic acid (not folic acid which is a no no) which also requires magnesium to be converted into methylfolate. My Natropath has therefore concluded that these two pathways are being blocked by lack of magnesium and has put me on high doses of magnesium to try to get these chemical processes moving properly. It is a highly technical area (if you listen to any of Ben Lynch’s lectures you can get an appreciation of this). You would really benefit if you could get the aid of someone who has been trained by Ben Lynch if possible. If you cant, I would recommend reading everything you can on mthfr.net (if you havnt already). MTHFR support Australia also has course in how to prepare for pregnancy with the mutation and they give details as to how to get the supplements right. I havnt done it as I am already a patient but that might be another option for you if you are interested.

  3. You can overdose on any vitamin supplement, especially B12 Cobalamin.

    You need to update your medical information before it ends up killing someone.

    1. If you’ve got a link to a new medical study in a peer-reviewed journal, please send it over and I’d be happy to take a look. I’ll update the information if the evidence is there.
      Many thanks

  4. From my understanding and what I have been reading throughout the internet is that b complex is not toxic, because it is water solutable, however, b12 has been known to stay in your system for many years. I just had mine checked again because I was deficient about 5 years ago, and have been supplementing with sublingual ever since. Now I am over 22, 000, yes that’s 22 thousand. But the doctors day the don’t for see any problems. However, not normal but I have been off and on sick since November 2015 with various illnesses, such as bronchisis and other minor issues, most noteably severe leg cramps.

  5. I went to a friend at a health food store. He suggested B12 5,000 MicroLingual, but said I should only take it 4 days in a row then skip a day. Why do I need to skip a day. These dissolve under the tongue.

    1. …my experience is that it can not only make you feel crap but can also cause megoblastic anemia if you are short on other things which can result in heart malfunction which is what happened to me when taken in combination with folate. Ben Lynch refers to it (in taken to the folate component) as being a bell shaped curve – you ca n feel great for a bit & then get really hammered by it. I suggest to anyone wanting to take this or having unusual b12 test readings – get tested for mthfr first….

  6. Dear , My mother age is 66. her B-12 was 250 4 month ago and took b-12 injection and supplement medicine now after 4 month her test came B-12 is 1022. please advise is it danger or not?

    1. Hi Anees,
      That’s a little bit higher than the normal range but it’s certainly not unheard of. As the article says, there is no evidence that high levels of vitamin B12 are dangerous, so I wouldn’t worry about it. For a 66 year old, having low levels of B12 is far more common and a much bigger problem. If you’re not happy, talk to your doctor, but everything sounds good to me! 🙂

  7. Jenni, can I ask if you’re only referring to supplements causing problems?
    Would b12 injections give a shock to the system, and how do you start slowly when the common practice is to give loading doses? Thank you for the info

    1. Hi,
      Sorry I wouldn’t know – I can only speak from personal experience & I’ve always had high b12 – just from diet which i assume is because I wasn’t methylating properly. Interestingly though – when I started taking methylfolate & methylcobalamin to kick start my methylation, I was dizzy & felt sea sick for nearly 6 months before I returned to normality. Increasingly, I just got back from a cruise the other week & no sea sickness at all for the first time. I know b12 shots are used for sea sickness – I’m now wondering if those affected by it might also suffer from a methylation impairment. Certainly something they’ll no doubt discover in the coming years.

  8. I started taking vitamin B 12 sublingual after I read it it might help loss of sense of smell and some taste you to viral cold. I haven’t Or tasted much for almost 3 years. I think the vitamin B 12 help although I’m not sure. My B12 levels were tested a couple days ago and it came up very high – greater than 6000. My doctor did not comment on this. My taste and smell got better after starting at 12 though I can’t prove it Was the B 12. All my other blood test were pretty normal except for genetic high cholesterol but my HDL good and my ratio 2.2 is good. I do have MS and I read it is good for myelin. Should I worry about this high B12 level?

    1. Hi Ellen,
      High B12 levels are not dangerous. The vitamin has extremely low toxicity and will not do you any harm. B12 is also water soluble, so any excess vitamin B12 will just be excreted in urine.
      A high B12 level is nothing to worry about. It just means that the supplements worked! There’s no need to keep taking B12 supplements when your levels are high, but keep an eye on things in case your B12 level drops again in future.
      Hope that helps!

      1. Thank you for you reply on high levels of B12. My doctor said he didn’t think it would hurt but probably wasn’t necessary to take so much. I’m trying to cut down anyway to 500mg to see if it makes a difference on being able to smell and taste. If I find it gets worse I’ll try adding 500mg back.

        1. 6000 x 8 = 48000 a day. Is that right? What are you taking that much for? And which kind of Vit. B12 is it? It may not be toxic ( I don’t know for sure)!but it might me a little waste of $! Check Google and ask if it might be toxic. It is water soluble but too much may not be doing any good.

    1. It probably won’t do you any serious harm but I can’t see why anyone would ever need to take anywhere near that much B12.

    2. What I wrote is a typo I was taking 5000 ( not 500)per day but I’ve cut back to just 5000 once a day.I only started it because the other group I belong to for those people who have trouble tasting and smelling said that vitamin B12 helped some of them. Turns out that some of my smell and taste did return so I started taking more B2 to see if it would even be better. And I’m not so sure whether it was a real cause-and-effect to begin with. I think my lack of smell and taste was due to a virus but then I’m not sure could’ve been done from MS which is your logical. But it happen so quickly after cold I still think it was a virus. And those take a long time if they’re going to come back at all. So I’m taking less now but nice taste smell are still intact not like they used to be but better than they were when I have no taste and smell.

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