Can B12 supplements have side effects?

B12 is one of the most important nutrients we can put into our bodies, but can we have too much of a good thing? 

Throughout popular culture there is the warning that too much of anything can have potentially negative effects. Remember the warnings in school about how too much carrot could turn you orange? Well, that’s partially true - the beta-carotenes found in carrots can indeed cause a condition which discolours your skin, only you’d have to eat a minimum of 10 carrots a day for at least a few weeks to see this happen. 

The same can be said of supplementation. There has been a rise in people taking more than a functional dose of certain supplements daily in the belief that they will be even more effective at keeping you healthy. Not only is this untrue - your body can only use what it can use and usually excretes or stores the rest - it can also have a negative effect. Supplements which are generally considered safe can have side effects in large doses, or when the upper limit is exceeded, especially when that nutrient can be stored in the body. 

With that being said, anyone can experience a side effect to a supplement they’ve never taken before, or whilst the body adapts to a new way of receiving nutrition. Here, we’ll examine the side effects of B12 supplementation, and what to look out for. 

Side effects from oral supplements: 

Oral supplements are generally considered to be safe, and to have no serious side effects. As ever, when taking a new supplement, you may experience some stomach issues whilst your body adjusts, but these should dissipate after just a few days. They include nausea and mild diarrhoea. If these side effects do not ease after a few days, then you should consult a healthcare professional. 

Megadoses (over 2,000mcg) of B12 may cause outbreaks of acne and rosacea, but these have to be extremely high doses (Brescoll and Daveluy, 2015). 

In fact, due to B12’s low toxicity and high upper limit, and the fact that it’s water soluble so we excrete whatever we don’t use, the majority of studies into the side effects of B12 have been performed on the side effects of using the high strength injections, and the interactions that B12 might have with medications used to treat other conditions. 

Side effects from injections:

Injections are sometimes prescribed for people who have long term conditions which affect the body’s ability to absorb, store, and use B12 - these can last for a person’s lifetime. On other occasions they are used in the short term to reverse and prevent severe deficiencies before the neurological impact caused by a lack of B12 becomes permanent (Wang et al., 2018). Typically, they contain a much higher dosage than oral supplementation and are required at regular intervals, such as monthly. When beginning injections to treat a B12 deficiency, as with any supplement, there may well be side effects, including: 

  • Mild diarrhoea
  • Itching or a rash on the skin, typically where the injection was given. 
  • Headache
  • Dizziness
  • Nausea / Vomiting
  • Tingling sensation in the hands and feet
  • Heart palpitations

(NHS Choices, 2019)

Other side effects and interactions with Vitamin B12:

However, these are not the only potential side effects from B12 supplementation. People who suffer from other conditions may experience side effects relating specifically to their pre-existing health issues. 

For example, people who are experiencing loss of kidney function due to diabetes may find that high strength B12 supplementation can cause the loss of function to speed up (House et al., 2010), and high dose supplements of B12 may also increase the risk of colon cancer in older people. (Oliai Araghi et al., 2019)

Side effects may also relate to other medications. When two substances have an effect on each other in the body, this is known as an interaction. Interactions with other medications can affect the ways in which B12 is absorbed by the body, or it can mean that your need for B12 increases. 

For example, one study has shown that B12 may interact with the drugs used in chemotherapy to treat breast cancer. Those who were taking B12 supplementation had an increased risk of their chemotherapy results being less favourable (Ambrosone et al., 2020).

Certain drugs, such as the diabetes drug metformin may have an impact on B12 absorption, which may require supplementation. When considering supplementation that has not been specifically recommended by a healthcare professional whilst on any other medication, make sure you check with them beforehand to prevent any unexpected interactions. 

Allergic reactions to B12: 

If you begin taking Vitamin B12 and notice any of the following symptoms, you should call the emergency services immediately as these are signs of a severe allergic reaction. Take the supplements with you so that the cause of the reaction can be identified quickly: 

  • Swelling of the face, tongue or throat
  • Difficulty swallowing
  • Difficulty breathing
  • Tightness in the chest

(NHS Choices, 2019)

What does this mean for my supplement? 

Supplementation with B12 is really important for those who are at a higher risk of deficiency, especially if that risk is related to an inability to absorb B12 properly from diet. This includes the elderly, pregnant and breastfeeding women, people who are taking the diabetes drug metformin, anyone who has had stomach or bowel surgery, or those who follow a strict vegan or vegetarian diet. 

Vivo Life’s Liquid B12 supplement contains 500mcg per serving, which is a functional dose containing the three most active forms of B12. This means that your body will be able to absorb the B12 it needs and excrete any surplus. It will also ensure that the body can keep its B12 stores stocked. The liquid form helps to bypass the digestive system and be absorbed quickly and efficiently which is more useful for people at risk of deficiency than taking a tablet based supplement which needs to be digested first. 


Brescoll, J. and Daveluy, S. (2015). A review of vitamin B12 in dermatology. American Journal of Clinical Dermatology, [online] 16(1), pp.27–33. doi:10.1007/s40257-014-0107-3.

House, A.A., Eliasziw, M., Cattran, D.C., Churchill, D.N., Oliver, M.J., Fine, A., Dresser, G.K. and Spence, J.D. (2010). Effect of B-Vitamin Therapy on Progression of Diabetic Nephropathy. JAMA, [online] 303(16), p.1603. doi:10.1001/jama.2010.490.

Wang, H., Li, L., Qin, L.L., Song, Y., Vidal-Alaball, J. and Liu, T.H. (2018). Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency. Cochrane Database of Systematic Reviews. doi:10.1002/14651858.cd004655.pub3.

Oliai Araghi, S., Kiefte-de Jong, J.C., van Dijk, S.C., Swart, K.M.A., van Laarhoven, H.W., van Schoor, N.M., de Groot, L.C.P.G.M., Lemmens, V., Stricker, B.H., Uitterlinden, A.G. and van der Velde, N. (2019). Folic Acid and Vitamin B12 Supplementation and the Risk of Cancer: Long-term Follow-up of the B Vitamins for the Prevention of Osteoporotic Fractures (B-PROOF) Trial. Cancer Epidemiology Biomarkers & Prevention, [online] 28(2), pp.275–282. doi:10.1158/1055-9965.epi-17-1198.

Ambrosone, C.B., Zirpoli, G.R., Hutson, A.D., McCann, W.E., McCann, S.E., Barlow, W.E., Kelly, K.M., Cannioto, R., Sucheston-Campbell, L.E., Hershman, D.L., Unger, J.M., Moore, H.C.F., Stewart, J.A., Isaacs, C., Hobday, T.J., Salim, M., Hortobagyi, G.N., Gralow, J.R., Budd, G.T. and Albain, K.S. (2020). Dietary Supplement Use During Chemotherapy and Survival Outcomes of Patients With Breast Cancer Enrolled in a Cooperative Group Clinical Trial (SWOG S0221). Journal of Clinical Oncology, 38(8), pp.804–814. doi:10.1200/jco.19.01203.

NHS Choices (2019). Hydroxocobalamin. [online] Available at: https://www.nhs.uk/medicines/hydroxocobalamin/.