Contributing to the grey area of information, the Institute of Medicine of the National Academies does not specify the maximum daily amount of vit B12 that could pose a health threat although it does warn against overdose because of the potential for adverse side effects.

Reported side effects include pulmonary edema or the accumulation of fluid in the lungs and congestive heart failure.

At this point, much speculation about what happens with B12 toxicity remains although the results at present raise concerns.

References:
Gastric Bypass Issues http://gastricbypassissues.com/complications/gastric-bypass-vitamin-b12-methylcobalamin-supplementation - accessed 6/20/12
LiveStrong.com http://www.livestrong.com/article/289103-vitamin-b12-deficiency-gastric-bypass-surgery/ - accessed 6/20/12
Natural Living Natural Healing http://www.naturalcuresmentor.com/b12-overdose/ - accessed 6/21/12
WebMD http://www.webmd.com/food-recipes/guide/vitamin-b12-deficiency-symptoms-causes - accessed 6/21/12

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You can read about my decision to have weight loss surgery back in 2003 and my journey to maintain a lifetime of obesity disease management since that time. My wish is to help you on your own journey of lifetime obesity disease management with shareposts along the way to help you navigate that journey successfully.