2.4 mcgs per day is the RDA for adults, generally (more is needed in some circumstances such as pregnancy). 100 mcg seems top be sufficient to reduce homocysteine for those with deficiency. Supplementation of vitamin B12 tends to be around 1,000 mcg (1 mg) of supplemental vitamin B12 to persons who are at risk for B12 insufficient or deficiency, mostly older individuals and vegans, though this dose is best only used for short periods with monitoring of levels so as to avoid excessive dosing and the potential risks.
Recent evidence has linked B vitamins (specifically B6 and B12) to an increased risk of lung cancer in men, especially in smokers. Here, we analyze the study and interview the lead author, Dr. Theodore Brasky.