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.
Vitamin 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.
Many 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:
- Numbness or tingling in the hands, arms and face
- Skin complaints e.g. acne, eczema or a rash
- Difficulty swallowing
- 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.
The 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.