B vitamins can only slow cognitive decline if the body also has enough omega-3 fatty acids https://ad-hoc-news.de/boerse/news/ueberblick/b-vitamine-neue-grenzwerte-sollen-demenz-vorbeugen/68437905 The body needs less B12 for blood formation than for repairing the nerve sheaths in the brain. For therapeutic use in nerve damage (neuropathy), 500 µg Vit B12 are required