It is an essential nutrient that helps the brain, nerves and blood work properly and which is found in meat, fish, eggs, dairy products and Marmite.
But it turns out that vitamin B12 could be causing acne, the skin disorder that results in spots, pimples and blackheads, scientists have found.
A new study has found that the vitamin changes the way bacteria behave on the skin of susceptible people.
Vegan diets could be the key to getting rid of spots, because B12 is mostly found in animal products. But be warned: not getting enough of the vitamin can lead to fatigue and a lack of energy.
The discovery could lead to new treatments for the distressing skin disorder, and shed light on why some people who take vitamin B12 supplements come out in spots.
While it was already known that taking the vitamin pills can result in acne flare-ups, no-one knew exactly why.
The new study solved the mystery by showing that the vitamin alters the metabolic activity of the skin bug responsible for acne, Propionibacterium acnes.
This causes the bacteria to secrete an inflammatory compound that in turn triggers pimples.
But the effect is not the same for everyone - some people are more vulnerable than others due to the complex interaction of bacteria and host.
Researchers led by Dr Huiyang Li, from the University of California, Los Angeles, write in the journal Science Translational Medicine: "Our findings suggest a new bacterial pathogenesis pathway in acne and provide one molecular explanation for the long-standing clinical observation that vitamin B12 supplementation leads to acne development in a subset of individuals.
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The paper adds that “interactions between the host and the skin … play essential roles in disease development”.
The research involved testing 10 volunteers who were all given vitamin B12 supplements. Within a week, just one of them broke out in spots.
To investigate what had happened the researchers collected bacterial samples from the participants' noses using deep-cleansing pore strips.
They discovered that excess B12 quenched the bacteria's own production of the vitamin, which had the effect of boosting levels of an inflammatory compound, porphyrin, released by the bugs. Porphyrin is known to drive the inflammatory skin response linked to acne.
The researchers added: "Future follow-up studies may potentially lead to the development of new therapeutics for this medically important disease."
The study proves the link between B12 and skin bacteria - but that doesn't mean people should stop taking B12, the researchers stress.
Vitamin B12 plays key roles in the normal functioning of the brain and nervous system, and red blood cell formation. Deficiency of the vitamin can lead to fatigue and lack of energy.
Perhaps the solution is to adopt a strict vegan diet – just make sure you avoid a certain divisive spread made out of yeast extract.
Additional reporting by Press Association