Presence And Formation Of Cobalamin Analogues In Multivitamin-Mineral Pills, The Journal of Clinical Investigation, October 1982

Various vitamins and minerals can “destroy” Cbl* under certain conditions.
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In our study, we have shown that the interaction of vitamin C, thiamine, and copper converts CN-Cbl** to a large number of Cbl analogues that appear to contain alterations in the corrin ring.
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We conclude that CN-cobalamin can be converted to potentially harmful cobalamin analogues by multivitamin-mineral interactions.

* Cbl = Cobalamin = Vitamin B12
** CN-Cbl = Cyanocobalamin, the type of vitamin B12 often found in multivitamins

Not only does the B12 in multivitamins become unavailable because it forms analogues (“20-90% of the cobalamin was present as cobalamin analogues”), but those compounds it forms can be harmful.

I found this study in Dr. Greger’s video:

Using multivitamins can be inefficient and counterproductive for the supplementation of Cbl [vitamin B12]. The Cbl can be degraded in the presence of vitamin C and copper with the formation of inactive by-products. These compounds can inhibit the transport system interacting with transporter proteins.

What to do? Buy a single-nutrient vitamin B12. Dr. Greger prefers cyanocobalamin because it’s more stable than methylcobalamin. Dr. McDougall prefers methylcobalamin. And either chew it or let it dissolve in your mouth, so it mixes with saliva.

This raises the question of weather you should avoid taking it with vitamin-C-containing foods or beverages … or thiamine or copper. I guess an empty stomach is best.