You evidently have heard something, but not understood it quite well, so are laboring under a great misunderstanding.
Vitamin B12 is vital to our health, so how so “dangerous”? It being water soluble, all excess will be peed out, so no risk of an overdose.
But, the lack of vitamin B12, classically seen in people having an autoimmune gastritis lacking gastric intrinsic factor needed to bind to vitamin B12 so it can be taken up in the last ileal loop,
causes severe anemia, also damage to tracts in the brain, spinal cord and peripheral nerves.
These last years we have seen another scourge causing serious neurological damage (paraplegia!) to young people recreationally repeatedly inhaling nitrous oxide AKA laughing gas AKA N20, this binds to vitamin B12 thus causes a serious depletion of vitamin B12 seriously, often not totallhy reversibly, damaging their spinal cord tracts leading to paraplegia (parasis or even paralysis of both legs).