I believe Pat Reeves has used high doses of B12 for medicinal reasons (due to the recent operations she's had it's unlikely she'll be one the board for a bit yet). My advice would be to stay away from mega-dosing with anything without having a reason for it. I often use high vit C if I suffer from a cold (1-3 grams daily), for a brief period, but that's about it for mega dosing. I say you run risks with high doses & also if you're body becomes accustomed to large doses, when you go back to a more normal intake you can suffer deficiency, or deficiency-like symptoms as your body is used to higher amounts.
Why do you want to use mega-doses of B12? What benefits do you want from taking it? If we can look at these, then maybe there are other ways to achieve it without the need for mega-dosing on anything.
[url=http://sports.groups.yahoo.com/group/veganbodybuilding/]Vegan Bodybuilding List[/url]
[url=http://veganbodybuilding.blogspot.com/]Vegan Bodybuilding Blog[/url]
[url=https://www.facebook.com/groups/23353662623/]Vegan Bodybuilding Facebook[/url]