B12 and B6 Vitamin Supplementation Linked to Cancer in New Study in General Health and Longevity Posted October 28, 2017 · Edited October 28, 2017 by Brett Black · Report reply Very high doses of a vitamin are deleterious to health - and water is wet :)... but seriously, yes, this is important. Some here might take at least B12 since a vegan might have problems getting it from diet. Presumably not super high doses, but it's a cautionary note. In my experience it's uncommon to find B12 tablets less than 100µg, which is over 40 times the RDI and almost twice the dose shown in this study to be linked to a doubling of lung cancer risk. My impression is that 1000µg B12 tablets may actually be the most commonly available dose - over 400 times the RDI. So it wouldn't surprise me to find people taking such doses.