What it is
Vitamin B12, properly called cobalamin, is an essential nutrient your body needs for healthy nerves, red blood cells and DNA, and it is the one nutrient a vegan diet cannot reliably supply from food alone. Here is the fact that clears up most of the confusion: B12 is not made by plants, and it is not made by animals either. It is built only by microbes, the bacteria and archaea, and animals come to carry it merely because those microbes live in their guts, their soil and their food. Commercial B12 follows the same logic on an industrial scale, and a scientific review puts it bluntly: cobalamin 'is produced industrially only by fermentation procedures using microorganisms', because the molecule is so intricate that chemical synthesis runs to roughly seventy steps and is hopelessly expensive. In other words, the B12 in a supplement or a carton of fortified oat milk was grown by bacteria in a fermentation tank, with no animal involved at any point.
Why this one matters
This entry is reassuring and important at the same time. Reassuring, because no vegan needs to fret that their B12 tablet is an animal product: every common form, whether it is labelled cyanocobalamin, methylcobalamin, adenosylcobalamin or hydroxocobalamin, is made by the same bacterial fermentation and is suitable for vegans. Important, because the very fact that B12 comes from microbes rather than plants means a vegan has to seek it out on purpose. The Vegan Society is direct about this, stating that the vitamin 'is made by micro-organisms and is not produced by plants' and that 'fortified foods and supplements are the only reliable sources of vitamin B12 for vegans'; the US National Institutes of Health likewise flags that people avoiding animal foods are at higher risk of deficiency unless they use fortified foods or supplements. The practical takeaway is simple and worth acting on: get your B12 from a supplement or reliably fortified foods such as plant milks, breakfast cereals or fortified nutritional yeast, and treat any claim that you can meet your needs from unwashed vegetables, spirulina or ordinary mushrooms with real caution.