Chronic Kidney Failure
People with chronic kidney failure do not detoxify cyanide as well as people with healthy kidneys. It’s thought that this may lead to nerve problems, especially in smokers (1). For this reason it is better for kidney patients to take a form of B12 other than cyanocobalamin.
Koyama et al. (1) suggest that patients on hemodialysis receive 500 µg of methylcobalamin intravenously after each dialysis. Vegans with kidney disease, whether or not they are on dialysis, should take a non-cyanocobalamin form of B12. These patients should talk to their health professionals about how much should be taken. 1,000 µg (1 mg) of methylcobalamin or adenosylcobalamin per day might be adequate.
Cyanide Metabolism Defects
Cobalamin has a strong attraction to cyanide. While being a natural chemical produced in the body, cyanide is toxic, and the body turns it into thiocyanate in order to excrete it. If this pathway is defective or overwhelmed through ingestion of too much cyanide (such as in smokers, or people in Nigeria who eat large amounts of cassava which is high in cyanide), the body may detoxify the cyanide by attaching it to cobalamin and then excreting the cyanocobalamin. Leber’s optic atrophy, tobacco-alcohol amblyopia, and other eye diseases can sometimes respond to high doses of hydroxocobalamin which serve to detoxify the cyanide. In these cases, there may be too much cyanide in the tissues (preventing conversion of cyanocobalamin to methylcobalamin or adenosylcobalamin) for cyanocobalamin supplements to be effective in maintaining B12 status (2). In such cases, a different form of B12 should be given (speak to your health professional) (3).
3. Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Washington, DC: National Academy Press; 2000.