I had this same question several months back. My understanding is that B12 donates its methyl group to homocysteine which then becomes methionine which is needed for detoxing.

One site I read said: Methylation is the process of recycling of homocysteine into methionine. It also talked about the addition of sulfate can often stimulate hg methlation while excessive amounts of sulfate can actually poison the hg meth. process by limiting the availability of hg to methylating bacteria.

The best site that I found with information on this is:
www.ever.ch/medizinwissen/b12hg.php

I also had stopped taking my B complex because of fear of methylating the hg, but I later decided to add it back in because of the importance for all the B vitamins. I didn't notice any ill effects. Mine is the cyanocobalamin (says from bacteria sources.) form.

This is another one of those unclear issues that goes along with hg poisoning. It sure can get frustrating can't it <img src="/ubbthreads/images/graemlins/tongue.gif" alt="" />