It was Christmas week 1990 in Oakland. Steven Bloom was wandering through The Lot, that timeless gathering of hippies that springs up in the parking lot before every Grateful Dead concert, when a Deadhead handed him a yellow flyer.
Because of the general safety of the vitamin, upper limits for normal intake do not exist. If your blood values for the vitamin fall below a range of 170 to 250 picograms per milliliter, you may develop a B12 deficiency.
For treatment of specific conditions which respond to Vit D, levels can be as high as 95 ng/ml. Symptoms of excess or toxicity have not been demonstrated at levels below 200 ng/ml. Old ranges can state that as little as 20 ng/ml is ok, which is the point at which serious symptoms of deficiency can begin.