Speaking ahead of 'Veganuary', when increasing numbers turn to a vegan diet each January, experts are warning that people need to make sure they get enough B12.
Adults need around 1.5 micrograms of B12 a day.
It is found in meat, fish, eggs and dairy products, but not in fruits, vegetables or grains - so those eating a vegan diet are advised to eat fortified foods, like cereals, or take supplements.
B12 deficiency, which can lead to nerve damage, tends to take three or four years to cause symptoms - usually first appearing as pins and needles in the hands or feet.
Tim Key, professor of epidemiology and deputy director of the Cancer Epidemiology unit at Oxford University, said: "You're not going to get B12 deficiency in Veganuary."
But Prof Key, a vegan for many years who takes B12 supplements himself, added: "If people become vegan because of that, and don't ever bother to read up about what you need to eat as a vegan, I would be worried they won't know about B12."