Hello, I m Dr. Branch, thanks for using Ask a Doctor . High vitamin B12 levels are not a good marker for any health problem, including cancer, and so I wouldn t be concerned about this by itself. Also, it sounds like you take a lot of supplemental Vitamin B12, which probably explains the high result. I don t think retesting is necessary, but if you were concerned, you could stop taking the supplements and recheck your levels to confirm they go back to normal range. I hope that helps, please let me know if you have any other questions about that, and I would be glad to discuss it with you further.