Methyl B12 and mercury - is it a problem?

There is an unfounded concern that you may read about that providing vitamin methyl B12 (methylcobalamin) in a supplement may result in it compounding with the mercury present in your tissues, resulting in presence of potentially toxic methyl mercury.

The short answer to this is it's not really a thing... you can go ahead and supplement B12 to support methylation and ignore it. It may be theoretically possible that, by taking B12, there will be metabolism of mercury into dimethylmercury. However how/whether this happens in humans would be subject to a number of variables (including methylation status of the individual, amount of B12 metabolised to methyl forms in tissues where mercury may be present). As methylcobalamin tends to be more prevalent in general circulation than in tissues, and is particularly less prevalent in tissues where mercury may be dumped (eg kidney) and where neurotoxicity could be a particular concern (eg. brain), the risks, if any, may well be minimised. Methylation of mercury via B12 into an organic form may happen in aquatic life forms and can result in dimethylmercury being provided in seafood diets, which is a concern from a dietary point of view. In humans, toxic levels might only be of theoretical concern if B12 substance is injected as a supplement at relatively high quantities. 

There seems no sound evidence that B12 (or any other methyl donor) supplementation will add to the effects of mercury toxicity in humans and in fact one has to take into account the overall benefits of supplementation for those undermethylating (eg. high homocysteine), including the detoxification properties of methylation. For example, in autism clinical benefits have been seen by supplementation of methyl donors even though those clients may be at risk of mercury toxicity.

It wouldn't actually help to use a different form of B12, because your body can always makes its own methyl B12 anyway and will do so as it's the most predominant form in circulation.

There is a secondary issue of methyl B12 coming into direct contact with mercury amalgam in the mouth when taken as a liquid (rather than forming a compound within the tissues). Again this is merely theoretical as the contact ‘opportunity’ from a time and area point of view is going to be minimal. Logistically the methyl B12 will be absorbed sublingually (especially if just a few drops) before being metabolised in this way. The benefit of the rapid plasma uptake sublingually would outweigh any risk of methyl mercury formation. There are also various factors that would come into play in terms of whether that methyl mercury is absorbed and gets into the nervous system etc, not least of which is gut status and bacteria (https://www.ncbi.nlm.nih.gov/pubmed/6524959). But if there is a concern with someone with fillings, use careful placement of the B12 drops!