After supplementing 5000 mcg Jarrow Methylcobalamin and 10 mg Source Naturals adenosylcobalamin (dibencozide) daily for a few months, my serum B12 level on last blood test is 5556 pg/ml. Is it bad too have it so high? How do we know we had enough? In December 2015 I tested 1425 pg/ml serum B12 and then serum methylmalonic acid was 18.9 µg/l (reference range <47). I didn't test it this time, because the methylmalonic acid test is a bit expensive. I am not sure if there is any difference in how I feel with with the 5556 pg/ml vs. 1425 pg/ml levels. How do I know I had enough? From http://www.medicaldaily.com/high-levels-vitamin-b12-may-be-linked-increased-cancer-risk-263123 "Significantly, the authors noted in their study that high plasma Cbl levels are probably not related to a patient’s consumption of foods or dietary supplements containing vitamin B12, as intake does not substantially increase levels in the blood. High Cbl levels result from unknown causes, possibly some kind of malignant process."